I have an odd but significant vibration occurring over 40 mph, mainly in 4th or 5th gear, but I can feel it in 3rd also.
It only happens when accelerating.
I've had the truck up, spun everything to about 95 mph, listened to the transmission / motor / and rear end.
I've deflected the driveshaft - doesn't look to be a U-joint.

I've got a billion other things to do right now and short of going out and buying myself a good reliable Honda Civic, I'm pained to chase this problem for another 45 days or so... It was enough that I took it into my local independent driveline shop this AM.

The shop thinks it's a motor mount that's being deflected during high torque. Looking at the mounts and torquing the motor (lock the brake, and put pressure on the driveline) - I get standard movement, nothing obvious.
There isn't any way to put as much torque on them as when it's on the road...

Basically has anyone blown a mount out of one of these trucks and have similar symptoms? This one is a new problem to me!

 

I'd think that if it were a mount, the u-joints, output shaft or even rear pinion you'd feel some vibration in every gear.
Which makes me wonder if you've got a tire out of balance or out of round. Any odd rear tire wear? Have you tried rotating the tires?

 

You don't say what kind of vehicle.

If 4WD you could remove the rear driveshaft and drive with the front end and see what that tells you.

 

Sorry - 1988 4runner.
It's not wheels/tires - it just started this behavior on the way home one day...

I could pull the rear DS.. It's definitely a "rear" of the vehicle deal though...