Did you put the dowel that guides the shift mechanism in backwards? The one that slides in from the rear and is retained by the hex plug?
When I rebuilt my AX-15 I remember having this issue at first, and I had installed that dowel backwards on reassembly. I flipped it and all was well.
98 Sahara 5.2L Magnum V8
Long Arms, 35x12.5 BFGs
My Magnum V8 Build Thread: