I bought a 1994 Ram 2500 with auto transmission last summer and it drove fine on the highway until a few weeks ago. It was on the interstate, and out of nowhere the truck suddenly started vibrating, bad enough that the view in the mirror was just a blur. Not a shimmy or death wobble; no effect on the steering.
Funny thing about this vibration, I can be driving along for ten minutes or half hour nice and smooth, no vibration -- then at some point the vibration just starts. Sometimes for no apparent reason, and sometimes going over a patch of rough pavement sets it off. Slowing down to 25 mph always makes it stop. Then I can speed back up and drive smooth for a while.
On a couple of occasions, going over a bump has actually made the vibration disappear.
It comes on, sooner or later, any time I go on the highway. It can even hit at speeds as low as 35 or 40 mph sometimes, but the faster I go, the sooner it will start in.

When the vibration hit that first time on the interstate, I took an exit, parked, and wedged a big block of wood between the muffler and the frame rail. The vibration came back on another highway trip a few days later but not as severe.
Now it vibrates more and more often. It starts to vibrate, at some point in the trip, every time I get on a highway.

U-joints?

 

Wheels out of balance ?
Shocks bad ?
Does it seem to be front or rear ?

 

2WD or 4X4?

Have you checked the carrier support in the middle of the driveshaft?

 

Could it be the large bearing in back of OD unit. The bearing can intermittently lock up and vibrate and make a heck of noise. Then it will free up and everything be peachy. Sometimes it will quit when downshifted to direct. If it quits when downshifted it is almost assuredlly the bearing. If no change, it does not eliminate the bearing or something in OD unit.

 

In answer to your questions:
It's 4WD
I had new snows put on the rear not long ago at a reputable shop in Middlebury VT, so I doubt the rear is out of balance. Can't really tell whether the vibration comes from the front or rear. The shocks look very old, but then the whole undercarriage is pretty crusty -- a lifetime of salty winter roads.
I rebuilt all the u-joints in the front driveshaft. I haven't looked at the rear driveshaft yet, but I will as soon as I get a chance.
It didn't occur to me to hit the OD shutoff button to see if it would get rid of the vibration, but since I have to slow down below 30 to get the vibrations to stop, the truck shifts out of OD on its own anyway.
Haven't drove the truck for a couple of days so no other info to report right now. Will report back when I find out anything new.

 

The type of vibration you have described is caused either by something physical changing, or a pretty severe harmonic. Something like a tire being out of balance will not suddenly start after a few minutes as if a switch were turned on. It can start however by changing speeds.

When you get it to stop, will it go for a while without restarting or does it restart right away when you get back up to speed. If it takes a while to restart, it is unlikely to be heat related but if it starts again immediately, it is likely. Some heat related things include brakes.

It sounds to me like you need to do some testing with shifting in and out of OD, in and out of 4X4, different speeds and stuff like that.

 

Just replaced a trans mount, [rubber gone], fixed my vibes

 

After I get the truck back up to speed it will drive for a while without the vibe.

 

I've run into this before, it was loose engine mounting bolts.

 

What he said! If you really want to see, take the hood off and watch what goes on "under the hood" when you drive. You will be shocked at how much things move around. Just make sure to scribe the mounting points on the hinges. I know because I'm an expert! cd

 

check trac bar