4wd clunks under pressure...

this just started happening about half an hour ago. I figured id post here and then work on it in hopes that people would leave me some clue when i get back. Its a 1998 jeep cherokee limited with a 4.5 inch lift on 31X10.5 (4.0). while i was climbing a small hill, the 4wd drive would "slip" and make a gnarly clunking sound, i had to keep it going or else all hell woulda broke loose. Then clunking persisted all the way up the hill, but when we made it to flat ground and the 4wd wasnt pulling the weight of the jeep up hill, it stopped. Anyways, im gonna go run the prelim checks, u-joints, fluids and driveshaft to the front diff. Anyways, throw me some ideas guys, if your in need of more info, let me know! tHANX A TON!!!
(also makes the noise when i put it in reverse in four wheel drive and give it gas)

Sounds like its the chain in your transfer case and the sproket. You would want to look in there first. I bet that is your problem.