I had a repair to the rear diff and they replaced a washer in the wrong place making the shaft and rear allignment off. Only one small washer and I had the same high speed results. At 90 Mph or above the rear made a terrible humming plus the lower speed vibrations. Good Luck, let us know what you find.
98 E430(sport), 99 ML 320 (sold) , 05 C 230(sport) Mark