2005 330xi auto rear end rumble

I have a 2005 330xi automatic. I got the car with 158,xxx and now has 164,5xx. About 2000 miles ago it got a rumble in the back end around 3-4 thousand rpms consistantly and constantly past 55mph almost exactly. And it rumbles when going around corners. I took it to the shop and the carrier bearing was torn so I got a brand new one. While it was there the mechanic noticed that the drive shaft looked to be from a junk yard. We had it put back together and the rumbling still occures. What else could this be rumbling? He also changed the gear oil in the rear because it was a bit low and black he said. No idea where to go with my current situation