I have an '02 Sierra 4x4 Z71 and for a while now, when I put the truck in drive there is a clunk coming from somewhere around the tranny/transfer/drive shaft area. If (while stopped) I change from drive to reverse (and/or vise versa) it also clunks as if there's play somewhere. I changed all my u-joints (both drive shafts) about 6 months ago. There doesn't seem to be any play in the driveline anywhere. One piece alum. drive shaft. Only fluid I haven't changed is the transfer case, which I plan on doing anyway soon as I have 121k miles on it.