I had the same problem around the same mileage but at much lower speeds, 3mph and up, the dealer told me it was the tires. I didn't believe them, I thought it was the Diff. Went to my tire dealer and got a second opinion, he felt it was the tires too. The Conties weren't worn out by any means, but were starting to get down there, and simply got noisy.
After replacing them, (with Bridgestone Blizzaks for winter!!) the car is now smooth and quiet again.
You may want to have it checked though before forking out $$$$$, a good mechanic should be able to pinpoint it without spending a whole lot.
2001 BMW 330I, Silver, 5 speeeeeed, Sports package, Fab ride!