JorgeA beat me to it. I was going to try and sound smart for once because I learned about that last semester in my engineering class. Darn it. Anyway, its the angle difference between your U-Joints. As the U-Joints rotate the input and output speeds change in a Sine-Wave fashion. In order to cancel this out, the U-Joint angles between the transfer case and differential should be as close as possible during normal operating conditions. Also, make sure the U-Joints on the drive shaft are 180 degrees offset from each other. Otherwise, you will just magnify the speed difference instead of canceling it out (If that is at all possible to change...not sure).

Mark