i currently have the same issue but for the front part.. and is not the bearing because it would sound at any speed plus i had the bearings checked by an svx tech at a dealer. (my sound is more like a wooble sound between 15mph and 25mph)

I talked to someone else in this board some time ago about this and it seems that it could be the driveshaft. My front U conection for the driveshaft could be worn out. I havent had a change to check upon this since my precious svx in atlanta and im in texas right now.. (long story but i bought it as my wedding car, for when i get married in 2 weeks)

Ill have that checked and fix afer the the wedding, but if you get it fixed sooner i woul dlike to know what was it and how was it fix.