Vibration no one can find

Call me insane but check your motor and transmission mounts.And make sure your harmonic balance looks good.

 

They don't call them freightshakers for nothing... Sorry too easy

Anyway likely it can't be fixed. You're experiencing an insufficiently damped harmonic vibration from the suspension and frame design.

If the vibration didn't exist when new you can look for worn parts in the suspension or frame, especially anything that pivots or absorbs energy. Also check for bolts that are "lose" or not torqued to spec (50 ft-lbs when they're supposed to be 100 will make a huge difference but you would never notice just by looking)

 

Along the lines of what I was thinking. But I have had worn mounts exacerbate the vibration.

 

this may be a dumb asnswer.you said you checked the u-joints,but waht about makeing sure the drive shafts are still balanced and trued?

 

Correct on that. A driveshaft can be slightly twisted and cannot be seen by eye. Take them out, go to a driveline shop, and have them put on the balance machine and ran up to 2400 rpm and checked for balance and trueness.

Also, driveline angle is as important as driveshafts. Ride height can change angles greatly, causing a harmonic type of vibration. Each yoke and driveshaft have to be within a certain tolerance of degree to operate smoothly.

I had a driver drop a clutch on my loaded truck. He slightly twisted the rear shaft. It looked perfect, but on the lathe it was horrible. I had a new one made that also vibrated. I took it back and watched as they ran it up to 1000 rpm and declared it OK. I insisted they go to 2400, as that was the it ran at. Around 2000 the lathe almost left the ground. Some extra weights on the shaft, and my truck was smooth as silk.

 

I had the exact same problem, only it was on a pickup truck. It drove me crazy. To make a long story short, they rebalanced my wheels,ended up putting twice as many weights that were originally on the wheels.
My brother in law has the same rig you have with well over a million miles on it. I seem to remember, he also had a vibration problem, I'll give him a call and post if he has any idea.

 

If that doesn't do it, check the harmonic balancer on the engine like Tazz mentioned. Your truck is old enough to need a new one if it's never been changed.