slight clunk is pretty normal. make sure your drive shaft u joints are good, and the trans mount is good, and you're fine. also could be worn out rign and pinion, but not likely.
if its a small clunk, its just the driveline shifting. the gears in the axle to the gears in the trans just all shifting momentum. from the trans to t case to axle there's a few gears going from one direction to another.
Check tranny/Engine mounts, U joint,Leaf spring bushing and last but not least if you have the cryco 8.25 it is a common find (dont ask me why) to find the pinion bearing worn out. causing slaping and clunking when loading the drivetrain.