I have a 93 and it has had three 4wd problems since it was purchased
new.  The first one was the automatic hubs had a technical service
bulletin on them and were replaced by the dealer for free.  I took it
in because the hubs were clicking (not working).  The second time the
4wd didn't work was right after the dealer replaced the o-rings on the
hub flanges and packed too much grease into the hubs.  Too much grease
causes the hubs to "hydralic" which keeps them from locking.  I
purchased new hubs from the dealer and installed them myself.  Not the
cheapest solution.  I did read on the net how they can be taken apart
and cleaned but elected not to do it.  The third time the 4wd didn't
work turned out to be a bad shift motor on the transfer case.  I
purchased a new one and replaced it.