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I had a very similar vibration on my 09 N54 after installed m3 subframe bushings. Vibration at high speed but only under acceleration. In my experience there's two possible hypothesis that could explain the vibrations.
1. When I installed the M3 bushings I F'd up and didn't pay attention to the clocking orientation of them. There's dots that are supposed to be pointed forward. I had the left side 90deg off from the right side. Fail. This "could" have created a stiffness difference between the left and right side of the car. Under load (acceleration) the rear subframe could possibly rotate very slightly relative to the body and mess with the angle between the drive shaft and diff causing the vibration.
2. When I bolted the subframe back to the body I didn't check to ensure the hole thru holes in the subframe bushings were concentric with the threaded holes on the body. This could also lead to a slight angle in the drive shaft where it meets the diff and create vibrations.
Once I realized I messed up the bushing clocking (item 1), I replaced the bushings with new ones and made sure the clocking was correct. Also I obsessed about getting the thru holes in the bushings to be concentric to the body treaded holes to ensure the subframe was square to the body. After doing this the vibration went away!
Not sure if you've worked on rear subframe recently, but that could likely have something to do with it. Also, if you decide to loosen & retighten anything up with the rear end I would highly recommend getting an alignment afterward.
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