What could cause bad vibration in higher gear on the RR of the car of an open diff.

I have the wheels up in the air and have already replaced the driveshaft with a known good one.

I get some near violent shaking coming from RR of car causing the muffler and dash to vibrate too.

I've recently replaced the rear wheel bearings with axle savers.

Didn't notice anything out of the ordinary with diff when taken apart.

All suggestions are appreciated

RT