Originally Posted by andrewwynn

Sounds like fat guibo concept was attempted. I have a feeling something having to to with the seal on the transfer case doesn't also a solid inserting of the splines into the transfer case. It's surprising to me that some X5 have this problem. It shouldn't be difficult to measure the amount of overlap of the spline by measuring the distance of the end of the shaft to whatever is the edge of the transfer case when it's installed compared to the measurement of the depth of the internal spline. Clearly there is very little overlap if that is the slipping going on.

How it isn't over an inch or two is simply stunning. Look into extended length drive shaft I'm sure the guys that sell them will explain the details. I've seen details of how somebody lengthened his own and also a kit with a two piece drive shaft so you can install the longer shaft without lowering either TC or diff.