Clunk in my rear diff

I get this clunk in the rear end of my MU, pretty sure its in the diff (lsd) only does it a coule of times usually whe I go around a right turn, go in reverse then I wont hear it again for a couple of days. Some times very loud and others times I can just hear it. Never hear it when I've been in 4wd or travelling down the hiway (thank god) Any help or suggestions most appreciated. Bruce