It has to do with the fact that the oil from the differential will run out. Eventhough there is a seal there, I wouldn't count on it holding a lot of oil in. IIRC, it has to do with the axle joints not quite beign in the same plane as the kingpin and when turning, the axle shaft will not stay in the center and allow the oil to bypass the seal. Then again, I may be completely off base I just remember reading something about it and that Randy at OEM was working on a solution to the problem. Whatever it might be.wouldn't it be a lot more simpile to get sealed u-joints and not even run boots?