Yep, if you are sure it's the rear end, nothing you can do but remove the cover and see what's going on. Nothing you can see from the outside (unless your driveshaft has fallen off, or the pinion nut).
Once you get the cover & DS off, clean out the housing, and check for anything obvious.
Check to see if the differential shaft locking bolt is still intact, and the differential shaft is still intact as well.
Then get some gear marking grease, and check the meshing of the gears.
Scott