My 99 TJ has been a daily driver with occasional light offroad to my fishing locations. I hardly ever use 4WD, but recently tried it. It entered 4WD and began making a terrible noise. It sounded like gears jumping over each other or chain jumping over a sprocket noise. The sound appeared to be coming from the transfer case area. Removal from 4WD stops the noise. It's consistent and repeatable. When it warm enough to try your suggestions, what steps would be best to locate source of the noise. I've driven the jeep a lot and it has over 105k miles. Thanks for any input.

get the tires off the ground put in in 4wd and in gear and crawl underneath

I got this thing on a lift today, No noise anywhere. Put on the ground and punched the gas and it's there. Appears to take a little torgue on the driveline.

well... if you have the spare time just drop the tcase and crack it open.. shouldn't take more than 3 hours with a buddy.. doing it alone can get you hurt...