I would just try to fix what you got first.
Down on the back of the front axle is the vacuum motor with a couple of vacuum hoses going to it. With the engine running and secured from rolling over you, feel for vacuum at one of the lines. Then place the transfer case in 4WD. feel for vacuum at the other line. If the vacuum switches, then that vacuum motor needs to come off and see if its just the sliding sleeve that's stuck, or the vacuum diaphragm is bad.
If there is no vacuum at either, next to check is the switch. Mounted on top of the T-case near the front output. Pull the four hose plug and check that vacuum is getting to it.
If taken care of, the vacuum CAD system is just fine. Its just another thing that gets neglected and after 100K+ miles needs a little servicing. When it finally "fails" your typical jeeper "engineer" decides that its junk and antes up for a better mousetrap.
Check yours out. Take the CAD motor off and see how it works and fix it. Its really easy once you see it open. They are easy to find cheap replacement parts for at boneyards off earlier XJs.