OK, since day one when I bought this thing in March it's done this. Right about 65-75 MPH the whole truck shakes bad enough to liquidate your kidneys. If you're brave enough to go 80mph it smooths out again as it does all other speeds.
1. Tires. It's on it's 3rd set, by virtue of much swapping around. All were balanced as is it's new set of Goodyears.
2. Wheels. My neighbor has an Explorer he drives very little and kindly let me swipe his wheels and tires to make sure my wheels weren't a problem.
3. An alignment. At the same time the new tires were put on.
4. A long inspection of the front balljoints and adjusting front wheel bearing play. Some of which was rechecked by the alignment guy.
5. Removed the 4wd "actuator" things from the automatic hubs to make sure one wasn't hanging up and spinning one axle all the time.
6. Put the rear end up on stands and ran it up to vibrations speed. Nothing. This also proved the transfer case was in fact staying in "2wd".
7. Despite this last test, I swapped the rear driveshaft with a known good one. (thanks again, neighbor)
8. New front shocks. Needed them anyway.
9. I've been trying to locate somebody with one of those "on car balancer" setups. No luck yet. Most tire people act like they've never heard of such. I know they exist, I've seen one in action.

It's annoying as hell, occurring right at normal freeway cruising speed. I can't take it, I either have to drive like grandma or risk a speeding ticket.
At one point I sort of thought it might be in the front end, but after the new tires it seems more like the entire vehicle is shaking.
This is all I know to do, nothing had any effect at all other than it's now smoother at other speeds with fresh tires. Have I missed something?

 

The same thing happened for me, I replaced or fixed stuff but it never went away. I just have to live with it.

 

Sounds like you have tried everything except one. i had a vibration in my 97 X and it was the front brake rotors. Maybe your neighbor can donate his to the cause.Hope this works for you.

 

Bad U-joints can cause that exact problem. I would give them a quick check, might find one side is completely dry.

 

Rotors were changed during the brake job it needed. No difference. I'd still like to try an "on car" balance though.
Bad u-joints? Please reread # 7. My neighbor's Explorer doesn't have the problem. He is currently driving around with my driveshaft and I am using his. No difference. Our Explorer vibrates, his doesn't.
Any more ideas? Anybody know what the Ford TSB "rear end stabilizer" thing looks like and if it would be of any help?.

 

Does the vibration get better or go away if you place the transmission in neutral (I assume you have auto trans) at the speed where the problem is worst? If so, look to loose torque converter bolts at the flex plate, loose engine-to-transmission bolts, bad torque converter, or bad harmonic balancer on the engine. Broken motor or trans or transfer case mounts can sometimes make things funny.

If placing in neutral does not help, then I would check the outer front CV joints. If they are improperly seated or worn, they can vibrate at speed, even with the front-end disengaged. (Does not apply obviously if you have the ability to disengage the front hubs - I don't know details of the 94 Explorer 4x4 system).

You might also try to get a look at all the NVH (noise-vibration-harshness) TSB's for your year and model. I know that certain 1996-97 Explorers had an improperly assembled front driveshaft/transfer case issue that caused vibration at higher speeds. The fix was replacement of the driveshaft and case, and it was covered under warranty at the time.

 

When I mentioned U-Joints, I was thinking about the ones on your front axleshafts connecting to the wheels (Gen 1 X's don't have CV joints). Ford made these ungreasable and it is quite common for them to dry up.

 

Ah, OK. But the FWD system is disabled. The axles aren't even presently connected to the hubs much less rotating. I had thought along the same lines at one point.

 

Some good thoughts, Duke. I would think converter problems would show themselves at particular RPM's rather than road speed. What the heck, I'll try the "neutral drop" as soon as practical. I'll try anything at this point.
Trans mount is new, it was replaced along with the transmission. Note that we bought this thing with a bad trans and never drove it until after the tranny was rebuilt. Although the tranny was rebuilt by a very reputable shop, I'll doublecheck the bolts and such involved. No way of knowing whether the previous owner had the vibration before the trans went out or not.

 

If the vibrations are felt in the seat and changes when you accelerate and decelerate, the flange from the transfer case to rear driveshaft was loose on my 93 4x4. Unbolt the driveshaft tighten and locktite the flange nut.

 

In general converter problems are indeed engine rpm related, but when the transmission is in a lower gear, the engine does not have to exert as much torque per degree of revolution on the transmission input shaft (it spins the input shaft faster, but does not necessarily twist it harder). The same effect is seen with a manual clutch, in which slippage (as evidence of a failing clutch) first often only occurs in high gear.

If the transmission has been out of the truck you might also try what Ford describes as realigning the mounting points. Loosen slightly but do not remove the engine, transmission, and transfer case mounting bolts. Slowly drive the truck on level ground forward and back about 5-10 feet. Then tighten the bolts with the truck still flat on the ground, suspension in effect.

 

Another thought - I have been focusing on your engine/trans because of your initial statement about shaking so severe it could damage kidneys. Shaking of this kind requires a lot of energy. Spinning parts have kinetic energy of rotation that is proportional to both the mass of the object being rotated and its speed of rotation. In general, driveshafts and hubs can vibrate a lot, and make whining and clunking noises. But they do not spin very fast and they don't weigh much. So they usually cause annoying but not earth-shaking vibrations.

Only the engine and the input side of the transmission have both the speed of rotation and the mass of components being spun to really shake the world. I know there can be exceptions, but in general this is true.

Steering problems can certainly make a truck shake violently, not so much due to speed as the large mass of the spinning tires/wheels/hubs. But a severe problem in this area usually causes severe shake of the steering wheel, and shimmy of the front end.

 

My 94 XLT is in the shop as we speak. They are repacking the bearings and said one side was a little loose. They said that the tires needed balanced.
I feel the shimmy at lower speeds over bumps etc. as well as above 55mph.
Do you feel the same thing?
I will post whether the problem has improved any.

 

More ideas! Great! If I can get the thing away from my wife long enough to work on it.
The vibration can't really be called "steering shimmy", which would be felt more in the steering wheel than the rest of the truck. And I've had few cars that DID shimmy.
The truck is fine to drive at any other speeds and is quite smooth. No clanks, grinding, or other noises that would point to bad components.
I had an F100 once that loosened it's axle pinion nut. I don't recalled the symptoms being anything like what I'm experiencing now but I will surely check all the nuts mentioned. No doubt a good idea anyway.
The "drivetrain realignment" is news to me. Sounds worth a try. The trans shop seemed very concerned about the flatness and concentricity of the bellhousing. So much so that they sent it out to a specialist who turned it on a lathe (a BIG lathe). They tell me that this is a surprisingly important and often neglected step in A4LD rebuilding. They never mentioned "aligning" everything. Though I installed the trans myself, I'll ask them about it.
Thanks guys. If anyone can add more, please do. If I get any kind of results I'll post them.

 

FYI - This is the ford description of "resetting" the mounts for my 1997 Explorer (Mountaineer, really).


Powertrain/Drivetrain Mount Neutralizing
WARNING :The electrical power to the air suspension system must be shut off prior to hoisting, jacking or towing an air suspension vehicle. This can be accomplished by turning off the air suspension switch. Failure to do so can result in unexpected inflation or deflation of the air springs, which can result in shifting of the vehicle during these operations.


<TABLE cellSpacing=2 cellPadding=1><TBODY><TR vAlign=top><TD>1.</TD><TD>Raise and support the vehicle. </TD></TR><TR vAlign=top><TD>2.</TD><TD>Loosen, but do not remove, the powertrain/drivetrain mount fasteners. </TD></TR><TR vAlign=top><TD>3.</TD><TD>Lower the vehicle. </TD></TR><TR vAlign=top><TD>4.</TD><TD>CAUTION</B> : Do not twist or strain the powertrain/drivetrain mounts.

Move the vehicle in forward and reverse (2-4 ft).

</TD></TR><TR vAlign=top><TD>5.</TD><TD>Raise and support the vehicle. </TD></TR><TR vAlign=top><TD>6.</TD><TD>Tighten the powertrain/drivetrain mount fasteners. </TD></TR><TR vAlign=top><TD>7.</TD><TD>Lower the vehicle. </TD></TR><TR vAlign=top><TD>8.</TD><TD>Test the system for normal operation.</TD></TR></TBODY></TABLE>