My 96 FZJ-80 with factory lockers has the same whine, started about 30,000 miles ago, removed front driveshaft, noise stayed the same, put in a heavier weight gear oil in the rear diff, noise became quieter. After ~30K miles the noise is still there, worse under load, goes away when coasting, comes back as I start up a rise in the highway. After talking to the Mud experts and a couple of diff rebuilders the consensus seems to be worn carrier bearings and/or pinion bearings in the rear diff. As it was explained to me the noise is caused not by the bearings themselves but by the mismatch in the pinion gear teeth and the ring gear teeth caused when the bearings lose their preload allowing the ring and pinion to move outside of their original (correct) positions when under load.