"clunk" going into reverse

Get under it with trans in N and wheels chocked or park brakes set tight. Use your hands to twist the driveshaft back and forth starting at front and work back to rear end. See if there is any difference in slack. Then put trans in park and see if trans yoke spline has slop. You should be able to tell where slop is coming from. If in rearend then just a matter of time before the BIG CLUNK.