'91 NP231 has 4-low but no 4-high

Anyone have any idea what could do this? Jeep shifts fine and you can hear the axle disconnect click over and the light comes on but in the 4-high position there is no power going to the front wheels. 4-low works great, pulls strong and makes no bad noises. Could this be linkage related or internal? Appreciate any input as we can't really look into it till we get the tranny back in my car.

Shift it into 4-Hi and crawl under and see if you can rotate the front shaft. It should be locked up with the rear driveshaft not allowing you to turn it. Do the same in 4-Lo just for a sanity check...

On the drivers side of the trans/Tcase is the linkage. It's possible that is out just enough to reach 4-lo but not 4-hi. You can lossen a nut on the shift linkage rod and slde it a round a bit.