I had a bad vibration on my 2001 330ci above 75mph. Turned out to be the rear differential bushing. Changed it just a few weeks ago and car is rock solid again. Worn bushing was allowing the rear dif to twist causing the half axles to tweak/not be square with the wheel hub / bearing.
Think about it...if the dif twists the CV joints on your half shafts are now loaded up and the car will vibrate at high speed....not good for dif or axles.
Get under the car and look at the bushing attached to your rear differential cover. Mine had a visible crack in it from 11 to 3 oclock position. Does the differential look to be straight up at 12 or is that huge bolt going through the dif bushing/dif cover pointing down a bit, toward the driver side, now go in reverse and look at it...the dif will be tweaked the opposite way.
Check out Pelican parts. I installed the OEM Lemfoerder brand...quite inexpensive but a mother to install without the proper removal and insertion tools.
Last edited by QAfred; 10-05-2012 at 08:48 PM.