Originally Posted by nicksbar
wouldn't the hum be constant if it was the bearing of diff?
the drive shaft is constantly turning
if it was a bearing ...yes,but im thinking its the shimming of the pinion.. maybe someone rebuilt the rear and didnt shim the pinion right,or assumed the shims he took out, would be the shims he put back.. not so.....
only under power is the pinion trying to rip away from the ring gear.. the driven force from the pinion against the ring gear is a twisting force due to the angle of the cut of the pinion gear, hence it trying to twist away from the ring gear under power, under de-acceleration or coasting, the effect would be reversed and the ring gear is trying to pull the pinion in close, again, due to the twist of the cut
as far as shimming, you need specified tools to set the backlash on the ring and pinion..
but... the FIRST thing you should do , is check the oil level!!
best way to do this is lift the rear of the jeep to get the pinion at a level angle, or close to it, then pop the plug off the diff cover, if oil comes out, youre good.. if not, fill till oil seeps out.. then quickly put the plug back
but im suprised you didnt check the oil level yet