Transmission or rear end problem

I have a 1999 Suburban with a 5.7. When stopped at a light, especially on a down-hill, when you take your foot off the brake to move to the accelerator there is a "clunk" sound. Can't tell if it's the transmission or coming from the rear end. Feels like a backlash from slack as you go from the stop to the forward motion. It doesn't do it all the time, only occasionally. The drive shaft is OK, the shocks are bolted tight. The transmission also seems to take a little more time when going from neutral or park to drive. It moves quickly from neutral or park to reverse, like it suppose to. Any advise?