This was a used low mileage original diff out of a donor vehicle. It wasn't rebuilt or a reman.. (which is why I don't think that's the problem.. (unless this new one was on its way out too..). But I can't see that being the case either as the x5 isn't known for blowing them up and they usually stand the test of time. Going to see if I can disable power being sent to the front wheels.. if I can that will tell me for certain that the problem isn't compatability between diffs and is in the rear end. In a few min going to pull fuse 46 and see if I can stop power going to the front wheels, if not try to disconnect servo on transfercase and see if that does it. I know I can't disconnect the front driveshaft. . So I hope this helps to isolate the issue.