You most likely have stripped splines where the drivehaft goes into the case. I fixed mine with a custom made shaft that has a 1" longer spline. Bmw only uses about 1/3 of the spline in the case, about a 5/8"-3/4". The seal goes bad and the splines start to rust and eventually wear down till they strip. I am going to be making replacement shafts on an exchange basis for around the same price as a new aftermarket shaft (that wont help unless you buy a reman. case) You will have to remove the case for a minute to install the new shaft because of the longer splines, but it will save you around $1000 for a reman. case and used stuff will probly do it again in the future. I did mine on a lift so it was pretty easy, you should be able to do it on some good tall jack stands. If you can change a water punp, you can do this, just have a helper when you slide the case back to install the shaft. My setup uses a good inch+ of clean spline and the 1/2 splines that are still left, way more than bmw used. I'm still waiting for my blanks to make another shaft, I've been testing the prototype (beating the crap out of the x5) for a few thousand miles with no problems. If your interested, you can call for more info 262-749-0689 8am-6pm cst.