Rear differential noise in reverse

I'm getting some rear differential 'clunking' noise when putting my 1999 Cherokee Sport in reverse when cold. It clears up as the car warms up. Any thoughts on to what this may be?

It could be a universal joint, check driveshaft for looseness. [email protected]

I have NEVER been in ANY rwd auto vehicle(car, truck, or suv) that didnt cluck(or make some other noise) when put into reverse. Its just clearances in the tranny that are excessive simply because the tranny is not at its operating temp.