Vibration after rear end install?

maybe

Is the pinion angle correct? If you install a new RE, and the nose is pitched up or down just a bit, the universal joints will start to rumble because beyond a certain angle (pinion angle) the joints have to speed up / slow down as they revolve, setting up a potential vibration.

You could put it up on jack stands and have someone outside the car as you slowly increase the speed and see if the source of the vibration can be seen. Go easy as you can get some backlash from the unloaded wheels, thru the clutch anti-chatter springs. On hi-po clutches, they don't like that kind of chattering.

You could just have a bad/damaged U-joint too. The driveshaft flange might not be tightened. Or it might have some trash/grit/etc under it preventing it from seating fully, which would cause wobbling and the vibration you are feeling.

And, of course, it is possible that you have a bent axle or any of several other damaged parts (bent pinion shaft, etc) that will cause this.