Slight vibration from rear of car

I picked up a ZHP a couple weeks ago and so far love it. They car has 150000 miles, but is in pretty good shape mechanically and cosmetically. The one thing that has really been bugging me about the car though is a vibration I have above 55 mph.

It is a very mild vibration that comes from the rear of the car. It is not noticeable until 55 mph and increases slightly up to about 70 where it seems to level off. If I go wide open with the throttle the vibration seems to lessen or go away completely (tough to tell over exhaust). If I let off the throttle and coast it gets a little worse. If I continue to coast it will lessen as the speed decreases till 55 when I can't really tell it's there.

I have done quite a bit of searching on the issue and taken the usual steps for fixing. So far I have replaced the following:

guibo
CSB
rear axles
rear wheel bearings
tried known good wheels/tires

All that is left that could be causing it is the driveshaft and rear diff. The rear diff was replaced by the previous owner about 2 years ago with a 3.46 unit with unknown mileage. He said the diff bushings were changed at the same time, but I do get a clunk from the rear with aggressive throttle applications or when sitting at a stop and engaging a gear. Diff bushings look fine, but definitely look older than 2 years.

I want the input of the people in the know before I go down the path of replacing the driveshaft or rear diff.

Just to make sure check that subframe because of that clunk...

Sent from my PC36100 using Bimmer App

Did you align the car? I can tell if my car's alignment is off or if a tire is low because the car will handle like crap and/or have a similar kind of vibration.

Take out drive shaft and tow to see if noises there.

Check all driveline mounts. Rear subframe area, transmission, and engine. Along side your compiled list.