There should be a round thin metal tag (about the size of a silver dollar) on the back of the t/case. If it's a 241, it'll read 241D on yours. If not, it'll have different numbers identifying it.
Rocket Scientist previously posted:
"Just did some work to the old Dodge & noticed a popping noise coming from the transfer case under a load & coasting or gear braking to a stop."
Reading your first sentence, I thought it made the noise all the time (2wd & 4wd). If the snap ring is broken, it'll make noise in both ranges. I just mentioned the most likely
issue with these t/cases so your issue could possibly be something deeper inside since you indicated it does it in 4wd only. For no harder than it is to remove the tail shaft housing, go ahead & yank it off just to see if the snap ring is intact or not then you'll know if you'll need to yank the whole thing out or not. If the snap ring is broken, that "fix-it" kit looks to be a good solution.