This problem has been getting worse since I bought the truck. I plan on replacing the U joints and getting new tires and my wheels balanced soon, hopefully thats where the problem is. Could be something other than the wheels/tires/u joints?

This is what's goin on:

The truck idles and revs just fine, its not a misfire or engine vibration, and the mounts look good.

The truck coasts in neutral or drive just fine when my foot doesnt touch the gas.

Whenever Im driving down the road and step on the gas, I can usually hear a 'clank' and then a smaller 'clank' when I take my foot off the gas. It also does this when I first put it into drive when parked.

When driving, if I step on the gas, a bad vibration starts. It feels just like when you drive over those divets they put in the concrete on the side of the highway that vibrates the car if you drive over them. Anyways, if I give it wide open throttle, the problem mostly goes away.

Like I said, the clank happens when the truck is put into gear from park, but it also happens when Im coasting then step on the gas (which is also the same time the vibration starts.)

 

Replace the U-joints.
Check both yokes for slop while you have the shaft out.

 

You didn't say what truck you have, but the larger trucks with the Ford Sterling 10.25 rearend have a problem with the nut coming loose that holds the yoke to the rearend. If your seal is leaking too, it's a sure sign it might be loose. You can usually tighten it up as tight as you can and it will hold for awhile, but the real problem is the splines wear on the yoke where it slides onto the input shaft of the rearend. Once they wear, they let the yoke walk back and forth a little bit, and this loosens the nut over time.

 

We do need to know a lot more about the truck. For instance, if it is a 4wd, many of which use a drive shaft with splines, the splines may be badly worn. So, when you pull the drive shaft to replace the u-joints you need to look it over very carefully.

 

Whoops. yes, it's a 1986 F250 4x4. Also, I believe it is the 10.25 rear axle, at first glance it looks bigger and heavier than a Dana 60, but not sure. My door sticker has C5 as the axle code and according to my 1986 shop manual it says it's a dana Limited slip?

Code: C5
Description: Dana Limited Slip
# Capacity: Over 8500
Ratio: 4.10

 

Ill try and post some better pics. It's dark outside and I only have a cell phone to take pictures.

 

BTW, what is a good brand of U Joint? and what is the difference between 1330, 1350 and 1310-WJ? Or would it be best to call a company like Moser or Strange?

 

I don't see a drain plug in the rear cover. That means you have the Ford/Sterling 10.25. Do some checking first, you may not need any u-joints. If you do, the local stores usually sell good name brand joints. You can go to one of their sites like Autozone's, and see what they call for on your truck.

 

I've had both of the above problems (loose pinion and bad splines) but your symptoms sound most like a worn tailshaft bushing to me.

As Dave suggests, check first.
Chock the wheels and put the truck in neutral to take any bind off the driveline.

Try to move the yokes in all directions.
They should not be able to move up, down, or side to side.
The rear should not be able to move in or out.

Hold onto each side of the U-joints and try to twist them each way.
If you can see or feel them moving in their cups they should be replaced.

Most any brand is fine if you keep them lubed.
Dana/Spicer joints would be OEM replacements.

Hope this helps.

 

Looks like you have good advice. But if you do have to pull the drive shaft, don't assume the splines are good just because you can't move things. On one of mine the PO's had apparently never lubed it and the grease was absolutely brick-hard. And the two pieces wouldn't move with respect to each other - until I took the shaft off and started replacing joints. Something I did caused the grease to let go and I realized the two parts wiggled. Badly. When I finally got them apart the splines were extremely bad, and that was apparently the cause of the vibration. I say apparently because I also replaced joints at the same time as having the splines replaced, so don't know for sure. But, the joints weren't very bad, so.....

 

Gary,
You have a splined telescoping driveshaft.
A 4x4 250 should have a slip yoke at the transfer case.
I had to replace the slip yoke in conjunction with my tailshaft bushing.
Fortunately the output shaft of my transfer case seemed okay.

This was a PITA.
The driveline shop I had my shaft rebuilt at screwed it up twice.
I finally just ordered the correct Spicer part from JBG.

 

So, the slip yoke doesn't use splines?

 

Oh yes, it is splined Gary.
The shaft shop got the right count but the wrong splines/length twice.
There is one spline that is 'keyed'.

The correct one for my 1356 T-case and 1350 U-joints is 31671.
If 1330 joints are used it is 31670.

 

Car-part shows a 86 f250 could have had 3 different transfer cases;

Warner
New Process, flange output
New Process, slip yoke output

 

Hmm.
I guess I haven't seen enough of them to know for sure.

Anyway, The OP can check for worn or loose yokes and universals.
The problem should be pretty apparent.