something else to check - gearbox in neutral and transfer in 2wd and wheels on ground - take hold of rear drive shaft and try to rotate, both directions. There should be very little play. The more play/movement the louder the diff whine will be. If there's lots of play it can be felt sometimes when changing from decelerating to accelerating, an obvious "clunk". Can be worn ring and/or pinion, or poor pre-load.