Originally Posted by BGTV8

The tailshaft is spigoted onto the flange on the nose of the diff.

Did you ensure there was no cr4p between the tailshaft flange and diff flange and they pulled down square.

Even a little bit of cr4p between the flanges will result in a vibration.

The other thing is whether all the bushes for diff mountings are snug - I dont recall whether there is a steel insert inside poly bush itself or not - if that was omitted and the diff can move on the bolt, you will get a clunk rather than a vibration though.

If have these and it certainly is noisier (a lot).

That is tailshaft side, driveshafts should also be snug and have no movement.

First things first, re-check all fasteners that were touched as first item of business