Put a temp gun on different areas of the rear axle ie.. differential, outer bearings, brakes/hubs, does the temp vary greatly from one side to the other? . Disk brakes? Front and/or rear? If yes check for a warped rotor or hanging shoes?
Disk front and drum rear.
I've inspected the brakes a few times in the last year but no luck finding the issue there. That said, I have to grease the backing plates on the rears so I'll take another look.