if it only has drive when diff lock is engaged then perhaps centre diff is gone, or some problem with transferring drive to front (ie broken axle??).
would it be when centre diff is unlocked it provides all drive to the axle that is easier to spin?? (ie open wheeler) if an axle is broken then no power transfers to wheels. it will just spin broken axle with all engine power going to easiest wheel.
(ae95 doesnt have LSD, i am assuming)
as soon as you lock centre diff then drive has to be forced to rear wheels causing drive, but if diff had choice it would just transfer all power to front.
does it make any loud noises when trying to drive with no diff lock??
im not 100% sure of operation and its been a while since i had to work on a 4wd and cant remember much of tradeschool days....but that maybe a cause.
at least you know its in front half, cause it still drives with the rear.