mine used to do this. On mine, the driveline was in a bind due to axlewrap. happened on take off or after stopping. The clunk was the driveshaft relieving the bind by moving on the splines in the xfer case (208). I was always able to band-aid it by pulling the driveshaft and greasing the splines a little so that it would slip more freely and not bind up. A traction bar probably would've fixed it. Since then I've switched axles and the truck is being rebuilt. If it still exhibits this afterwards, I'll get a traction bar.