I have seen some similar topics, here, but not what I consider the same:

1999 F250 SD, 129k miles, most of it with a trailer attached. Out of the blue, the 6 speed stick will start to vibrate back and forth, and then the whole truck will shake. This has happened at almost any speed over 30mph. There is no noticable tendency for the wheels to shake -- it doesn't seem to be a steering problem. If it was, I wouldn't be here to type this.

The only way I have found to stop it is to slow down significantly from whatever speed I was running. I thought it was being started by a rough spot in the road. Well, that is not the case. It will start with just a little bit of normal road surface roughness.

Action taken: Replaced all tires (Uniroyal Laredo, AWP). Rotated tires several times. Repacked and torqued wheel bearings. Increased air pressure in tires. Decreased air pressure in tires. Had tires re-balanced. Put in the Ford house for front end check up.(nothing found).

This really doesn't feel like a suspension problem. It has happened when running empty, when towing 6000 pounds and when towing 12000 pounds. (I know, 12k is pushing the envelope).

Any help would be appreciated, just keep in mind that I am not rich and can't take it to the Ford Hou$e for every little thing. :0)

Thanks,
bearswatchin'

 

Just guessing here. Motor mounts, transmission mounts, bolts loose holding the transmission to the engine, or maybe the midshaft (carrier) bearing. Just a thought.
Joe

 

do the 99s have a dual mass flywheel? I know the older standard trannys did, and when the flywheel started to go it caused violent shaking. I know the dual mass was phased out, I just don't remember when.

 

Thanks, guys. I'll give it a wrench test and see about loose bolts.

 

Sounds like binding U-joints to me. Or what Joe-M says. Loose crossmember in the tail of the trans.
Another possibility is an engine miss, but then the tone of the engine would change and I am sure that you would have picked up on that.

 

No change in engine tone. Just the shaking. If it was a side to side shake, it would throw me off the road. It is that violent.

I intend to get under the truck and check for loose bolts, nuts and such.

 

One more question, is this a 4x4? If it is, then this could be your trac-bar bushings. What you describe almost sounds like the "death wobble" experienced on 4x4s when the track bar bushings are out, and the axle starts hopping around under the truck. But that won't be the problem if your truck is a 2x4.

 

Not a 4X4. Thanks for the input, anyhow.