These don't seem to fail often. I would verify the pump is in fact broken and not just a leak in the system. stealerships are more desperate than ever before - don't trust them....
Sounds like you have a leak in the auto-hub system. Start at vac lines to hubs, replace hose as necessary. Then follow those lines to pass-side fender well (engine compartment). There is a solenoid valve for hubs near vac pump.
I doubt your hubs actually locked in 4wd. If you can spin the front driveshaft or either axleshaft w/trans in neutral - hubs not locked. You can also jack one front wheel at a time and if tire spins while in 4wd - not locked...
After verifying vac IS going to hubs, check hubs following these directions:
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