1999 4x4 Ranger clunking

[D37mike]

So my ranger makes a very loud clunk noise in 4x4 Low. It only occurs in 4x4 low and only when the throttle is open. The U-joints have been replaced so I know they aren't the culprit. I have looked at the transmission and the mounts look fine. There were no metal chips in the transmission or the transfer case oil. I've crawled under the truck and I don't see anything that is hitting. So my question:what the heck is it? It feels like something that is hitting but I don't know what.