A bent rim, wheel out of balance, bent driveshaft, excessive pinion angle, or bad u-joint can also do this. A balancing machine will bring to light bent rims and balance issues. If you have someone who can ride alongside and behind you in another car, they can look for wheel hop, wobble, out of round or other issues.

Is there any noise associated with the vibration? I had bad spider gears in one diff and it growled with a vibration. Bad axle bearings will also cause a rumble or vibration, you can find these with the rear axle on jackstands.

You can isolate driveshaft/u-joint from engine/trans simply by pushing the clutch to the floor to see if the vibration stops. From the sounds of it, it will make no difference.