[burnin oil]

Clunk may not of come from the tcase. Odds are that if the bearings are not whinning then you are fine. If it where mine I would top it off with cheap oil then drop both drive shafts off the case. Fire the truck up in gear and crawl under it and see if any odd noises are heard. After that I would drain the case and see if any metal chunks come rolling out. If you never drained the fluid previously and the case was low from the previous owner the oil will probably be pretty nasty. Change it out and just keep an eye on it from there.