this wil be differential number 3. wtf.
original was number one, a replacement when i bought the car was two. and this one will be three.
any ideas on gear ratio for a 2005 525i manual? 3.23? does the diff have to be transmission specific?

Sounds like the same problem on mine.
 

Not to Hijack but I have an 07 530xi with 38-ishK miles on it and Im getting the same thing. Its got such low mileage I just couldn't believe it was a differential or even the flex disc and bearing. It is driving me crazy. Was thinking of having the wheels balanced first. The wheels have been repaired....previously and Im assuming its that but I was wondering if anyone had any input on it or if its even remotely likely that there is something mechanically wrong with it.

Bill, did you find out what was causing the vibration?

What I do not understand is the fact that you said that you tried 3 different sets of rims... Are all your rims bent???

An harmonic vibration is usually caused by tires or rims. Every time I had an harmonic vibration that I felt in the rear end, the tires were the issue.

I used to have this vibration when i get the car, car come with aftermarket wheels and had a bad vibration up to 60 MPH i just bought a set of aluminium hub centrics to center the wheels and vibration is gone maybe that can be your problem.