Take care of the problem before it gets worse and breaks other components causing your repair bill to go up. Slide yourself under the truck and rotate driveshaft to look for play in u-joints. You can do this with the truck parked or if they arnt that bad yet you may need someone to put the truck from reverse to drive while you view the u-joints. Be careful to place your body out of the way when doing this.
Thanks for the reply, I have actually checked the U-Joints and found the clunk is internal and is caused by the ring to pinion gap which seems to be excessive, taking it to the chevy shop friday/still under warranty. Once had this problem with my 72 blazer had to replace the crush sleeve and associated parts, this fixed the clunk.:)