[AZKen]

I suppose you are saying the "Plug Welds" broke, if there are any, allowing the tube to spin on the diff housing. The tube is pressed in and can have various welds or pins.

The diff will rotate due to torque a little bit during acceleration as springs flex.

If it was still rotated when stopped, then maybe Ubolts are lose.... or you have done a modification to the pinion angle.... or Ujoints.....or a combo of those. But you say perch was still at level to floor. Did perch weld break?

Those don't break/spin unles extreme 4X4'ing.

Also, did you just do some work back there? Something was done. New Ubolts, painted diff and older shocks with new bolts? Any geometry changes? Monster tires?