The vacuum solenoids are near the right fender. There is a vacuum harness that runs to the wheels. You have a leak somewhere between the solenoids and the hubs. Pull the vacuum lines off at the hubs and plug them. Try the AC again when you ask for 4X4. If it works ok it's the hubs. If it still goes to defrost you have a leak somewhere in between. Just have to trace it down.
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