Just when you think all the homework is done and everything has been upgraded to be super strong, the only original parts left in the drivetrain begin to fail!
The diff flange bolts were original, and believe it or not they started to stretch and get loose, so that eventually they got finger tight and the tailshaft started to vibrate strongly.
Replaced the original 10.9 hex bolts with 12.9 allen bolts and put new shakeproof nuts on. Now the WHOLE drivetrain us upgraded. Mutter ...