I had intermittent problems with my 1990 NP242 which sound identical to yours, although it wasn't consistent..sometimes 4wd would work flawlessly and other times I would get the same bang/losing power to the front wheels that you describe (in both full time and 4hi/4lo). I had originally suspected that the chain had stretched, but since it was only doing it once and a while I decided that the problem might be related to a shift fork or something else and not the chain since it didn't seem to matter if I was under load or just coasting down a gravel road.
Anyhow, one day it was acting up, popping out of 4hi and banging away so I gave it a ridiculously hard flogging and with one final 'bang!' it seemed to have fixed itself somehow, I havn't had any major problems since. Once in a blue moon I'll shift into part time/full time and feel that it hasn't entirely 'engaged' so I just shift back into 2wd and try again, usually the second time does the trick and I'm happily on my way. To be honest I think I had this problem since I bought the jeep a few years ago so I don't attribute it to any other damage happening. I just completed an 800km trip halfway across the province in full-time the entire way because the highway conditions were so iffy and I had no problems with it at all.
I'm not too sure what I'm getting at here, I mean..I don't really want to recommend that you flog your Jeep too hard, but I find a little 'tough love' has fixed all kindsa problems with mine over time. If I were you I would start by checking the fluid level, next I would try adjusting the TC linkage and if that doesn't fix it, start looking at the shift forks or chain next.
Good luck and let us know what you find, I'm interested to hear what the problem is just in case mine starts acting up again!