99 np231 front output slipping

Bought a 99 for $400 to use trans, tcase and axles for 4wd swap. Tcase linkage was out of Jeep but in 2 high it drove around and was quiet. Have case out and on bench and when 4wd is engaged (high or low) the front output slips (turning by hand) and when in 2wd and front turned by hand there's clack....clack about every 1/2 to 3/4 turn
Admittedly I have not owned or worked on a 231in 9 years so I'm kinda looking for thouhts before I 1) open it up or 2) buy a reman or source and hopefully good used case.

Either it is actually a 242, or more likely the chain has stretched.

It's tag says 231. In 4 low (or 4 high part time on 242) I wouldn't expect it to slip unless something was damaged.

Well there could be several things going on, but none of them will be obvious until you split the case.