STOP!!
Don't take it to bits yet!
The gearbox will be LT77, so the transfer box should be LT230 - I think.
Open the middle under-seat panel to get light and access.
On the right side top of the gearbox you will JUST be able to see the linkages that operate the Hi-Lo ranges, and that lock the centre diff.
Before you do anything else, make SURE that all the bits of these linkages are completely free to move, and that they DO move. NO force should be needed.
DO NOT USE FORCE! You will certainly bend things in ways they shouldn't be bent and it may never work again.
The linkage you are looking for is the one that pulls and pushes a short rod that points back and down, and attaches to a small flat lever at each end.
If need be detach the lever at one end, and see if that allows you to properly enagage High range by poking the lower lever towards the front. The rod is slightly adjustable at the bottom end, and I suggest that if you decide to sort it, you take the rod off, clean and copper-grease the adjusting threads, and put it back WHILE THE BOX IS IN Hi-Range. That way you can be sure to have the Hi-Lo lever getting it far enough into Hi to make it stay there. You can test drive it with the rod off to make sure it stays in Hi-Range.
Email me if you want a copy of the workshop manual diagram, which won't paste into this reply.
CharlesY
CYoung1097@aol.com