I think it is called a double compound angle and you will get a vibe at about 25 mph, so you have to go faster or slower than that, the transfer case also has more twisting on it so to speak so the driveline angle is trying to rip it out of the mounts more is the only way that I know how to put it, I drive a friend of mines Heep that has the same thing going on with a narrow Dana 44 and it loosens up the rear yoke once and a while and is hard on u joints other than that I have put allot of miles on it with a vortec doing 80 mph so there ya go and also his drive shaft is under 12" in length.