vibration

How about the front cab mounts? The rubber bushing has a steel sleeve in it. Any evidence of slack? Try a pry bar and see if moves.

 

Sorry, but I have to ask. You do know that the rear engine mounts are also the front transmission mounts, right?

 

The bolts that go into the bell housing may be tight, and the piece that's bolted to the frame may be as well, but the rubber in between them- that's another matter entirely. It won't show as a big gap, just the rubber either split or lifting off the metal, ever so slightly.

And under load- like going 50 up a hill- full throttle. You'll feel it.

 

There is a rule. If it feels like a transmission vibration, it probably isn't. Every driveline vibration will be felt by the driver as a tranny vibrate because you sit directly above it and the vibration travels up the driveline vibrating the whole thing.

As well as other thing stated..... I would go under the truck on your back and push up on the drive shaft at the steady bearing and see if there is excessive movement.

Check each u joint for Shiney metal as a sign of a loose u joint.

Where the drive shafts spline into the yoke make sure there is no play and there may be a grease spot on the actual drive shaft there that shiuld be packed full (on my columbia that's the only grease spot I have) on both shafts.

Make sure your ride height is proper and your leveling valve didn't break or bend. (I screwed my transmission and u joints because of this one)