Drivetrain vibration

When accelerating I am getting a severe vibration/bind in the drivetrain from 20 to 36 mph. Above that it goes away. RPMs stay smooth, shifts are smooth, no abnormal tod lights (work as they are supposed to), no strange noises other than the sound of the vibration. Front end has manuel hubs and they are unlocked. Trans mounts are tight. Thinking trans. What else is a possibility?

Does it happen at the same speed no matter what gear it's in, or does it happen at the same RPM if it's in different gears?

Same speed. RPMS make no difference nor does the gear. I have shifted it down and manually shifted it and it does the same regardless of RPM. Only thing I have not done is try it in 4lo...will do that next time I take it out near some dirt.

I'd check the u-joints in the rear drive shaft.

Wheel balance and bearings as well.

U joints are tight...no play. Will grease them to make sure.
Pretty sure it's not wheel bearings. It is more of a binding type vibration. All wheel weights are still in tact.

Have you tested for this vibration with the hubs locked? Could just be from the halfshafts or tcase spinning without load.

I've been driving with the hubs unlocked for the last 8 years.

Sure but things can wear out after 8 years... like the chain in the t-case. Maybe it will even get worse with the hubs locked and help you find the issue.

Clutches can cause shutter. The equivalent in an auto is the torque converter...which is one option. i don't know how to check this. Though you probably need it on a lift and listen with a stethoscope as you run up wheel speed (off the ground).

Another idea is a motor mount. Check for engine play when you rev up in park. Or put foot on brake in drive and have someone else look at engine shifting.

On mine, the u-joints weren't so much "worn out" as binding as they rotated. You could hear them at hwy speed. you can probably rule this out.

Places like NTB and/or Certified Transmission perform diagnosis for free! :-)

New info...
Took it out today and put it in 4lo...no vibration.
I can feel the vibration transmitting more through the tcase shifter than the trans shifter.
No vibration transmits through the steering wheel.

I think I'm going to drain the tcase and check the fluid and put new fluid in and see if that does anything.

Thanks for all the suggestions. Will keep hunting down the problem.

Whoops, wrong thread

Did you grease the slip joint on the drive shaft too when greasing the u-joints?

Yep, all 3 zerks are freshly greased.