GM Repair Problems? Ask GM Repair Expert Now.
Is it just sporadic, or just for a second, or does it vibrate constantly when you maintain that speed? And it goes away below and above that speed?
it's possible that your prop shaft (from the transfer case to the rear differential) either is developing a worn carrier bearing, or it may be unbalanced slightly. You can try unbolting it and rotating it a quarter turn relative to the differential (mark the original orientation so you can put it back if wanted). Or you can put weight onto the shaft to see if it changes the vibration at all, if you use a hose clamp (the screw fitting is the weight) then you can move it around the shaft forward or back, or rotate it around the shaft to change location and see if it changes.
It seems like, since it is based on speed and not on engine RPM, then it is the wheels, differential, or the prop shaft- the prop shaft being the most likely suspect. The carrier bearing can sometimes be beaten off with a hammer, and a new one pressed (beaten) into place. You would have to remove the shaft to do so.
As for putting a weight on, I would inspect and see if there are weights welded onto the shaft (square or rectangular blocks) and maybe start opposite them.
Don't make it too complex- just try one on one end of the shaft, see if that changes anything, and every time you go out move it around the shaft slightly, and then try the same process at the other end.
You can take the shaft completely off and send it out to a shop to see if it is balanced, they have specialized equipment to spin it up on similar to a tire balancing machine.