Could be a u-joint. If the diff is full of mud water, buy some Blue or Black RTV and some 80w90 diff fluid, pop the cover and drain the old fluid. If it's full of mud water, then you'll know what the problem was and pray it didn't destroy any of the bearings. Before you do that, however, crawl underneath the jeep and check both of the drive shafts for excessive play. That could indicate a ujoint.