yeah coyote is right, alot, a whole bunch of those yj's have that problem, i think in 93 they stoped using vacume actuated axle disconnect type front axle. it really is a dumb idea, but anyway. i am sure your tcase is fine, there are 4 vacume lines coming out of a "switch"
on top of your tcase, yellow green red and blue, yellow and green and myabe red go to your front axle, blue connects to your switch in your dash, beleive it or not that is vacume actuated to. so that little blue light is triggered by vacume pressure, anyway, those lines are prone to cracking, tearing, just not working. it is also possible that the shift motor on the front ale is bad too, those forks are pretty weak. pull the lines off the fornt axle (easy) and have someone put it in 4 wheel drive while its running and check and see if you have a vacume or not. that will help there, trace the lines if you have nothing than it is a switch, if you have a leak, replace the line. if you have vacume and it won't shift, replace the motor (sixty dollars) but keep on the look out for cable lock conversions, if your more interested in manually locking the front with a cable i can send you a link, but anyway i hope that helps.good luck