on my old 2000 with the j shift, my 4wd was not working correct and it was winter and snow and needed it working, I was testing all kinds of stuff, then someone on the forum suggested it might be the little one way valve that supplies the vacum to the system. Its blue ( if original), right next to the upper intake manifold, betweeen it and the radiator.
the way I tested was to take it off and blow into it from both directions. It should only work in one direction. mine did not work in either. No one had one in stock, so I just drilled it out ( then not a one way) I guess you could just get a vacum splice fitting and not drill it out....but I did that until I was able to get a new one. That seemed to gett the system working, unless I bumped something and it was all coincidence. But after getting the new one in, the system did not let me down and all worked as it should.
months later when I was at a pick and pull I grabbed the entire vacum system out of a car, including the diff actuator for spares...for $25
on my current 2000, i had no dash light telling me I was in 4wd, it was the bulb. Its improtant to exercise the systems. The P.O. of this one had never used the locker, and it took many attempts and then using it a lot off and on to get it to work correct. Now I make sure I run both systems a little every few weeks, as I will need them in a few months when the snow hits.