I have: an NP231 Transfer Case on my 88 Jeep Wranlger YJ with 6 cylinder & Peugeot Manual Transmission.

2 wheel drive and 4 low wheel drive work just fine, but no 4 high. The stick clicks into 4 low and it completely engages, but when I try to put it into 4 high it feels like something is blocking the shifter.

Anybody know what may be causing this problem other than vacuum lines? It's not a vacuum problem.

Thanks!

idk if this will help or not but i had a problem similar on mine only it was with the auto trans shifter, it would not go into park or first and it only did it a really random times, finally i decided to pull off the cover to the shifter and found that there was a random bolt in there that would vibrate into a certain spot and keep the shifter from moving into park and first, really simple fix which i should have done the first time it happened but i waited til like the 3rd or 4th time and felt so stupid, take a look around something might actually be blocking it

Sounds like your transfercase linkage need to be adjusted. If it is off a bit then you will not be able to shift the TC into the mode at either end of the shift lever push/pull. To check it out do this:
1) Go under the Jeep and find the shift linkage that is attached to the transfercase. On the rod part of the linkage is a nut you must locate.
2) Loosen this nut and move it around on the shaft until you can tighten in and climb back into the Jeep and get shifted into 4low.
3) Once the TC 4low loosen the nut again. Climb back into the Jeep and position the shift lever about a 1/16" to 1/8" from the bottom 2low shift setting.
4) Go back under the Jeep and tighten the nut real good.
5) Climb into Jeep and Verify shifting for all TC modes.

Also check the nut on the shift linkage where it attaches to the TC. This could be loose too. Good luck getting er' done!