If it is JUST 4low, it is possible you are traveling faster then ~30 mph, and the tcase is doing its jet turbine impression. if that is the case, change the tcase fluid, and slow down. if it also does it in 4hi, the front diff and cv joints are suspect also. the bearings are alwayse spinning when you are moving.. but repacking them can't hurt, may even pick up a little fuel econony if they are on the dry / dirty side. make sure there is not mud / other crap stuck on the front driveshaft, or the 2 front cv-shafts. they don't normally spin, and if they are caked they may be out of balance. first thing to check is if it also does it in 4hi at the same speeds.