Mossy500

I've had my xj a few months and I can only get it to engage full time 4wd. The previous owner didn't know if 4wd worked since the front drive line was behind the seat when I got it. When I try to go into part time high I believe it engages full time high. When I try to shift to part time low, nothing engages and seems to remain in the neutral position.
I've heard I may need to adjust my linkage, but I'm not sure how. Any other suggestions are appreciated.
87 xj limited.