sounds like a driveshaft u-joint. jack up the truck so rear wheels are off the ground and check for play in the joint. If there is any, replace it
I think only the reg cab with longer beds still use a two piece with a center carrier bearing assembly, but I do agree, sounds like a drive shaft issue.I believe that Mr. Chips and Ukrkoz have you on the right track, so I would definitely proceed as they described. Along with that, although I think they have been discontinued, and don't know if they were still using them in 2000, or not, check to see if your rear drive shaft is one piece or two, and, if two, check that all three u-joints are synched, or oriented the same. Also, my brother had a GM pickup sometime around then that had a composite driveshaft, which he had to replace, due to an incurable imbalance.