Well knowing that you have a YJ and therefore have a CAD axle would make you savvy. Your vacuum actuator or the vacuum switch at the Tcase or possibly the vacuum lines to either or both are faulty. Start with checking the vacuum lines where they pass over the exhaust. They may have come in contact with the exhaust pipe and burned through. Fix as necessary. If they are ok then move to the actuator on the passenger side axle tube. Make sure it moves like it should. Last, I'd try the switch on the driver side of the Tcase. It's got 4 vacuum lines going to it. It needs to work for you to engage your front axle.
'88 YJ with SOA and SYE and a bunch of custom crap from BWF in Chesapeake.
'97 XJ on 8" lift, double triangulated rear, long arm 3 link front, Eaton/60 with 5.14 gears.