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Hello, if the vents are working fine while the engine is running it has good vacuum. On the front axle is a disconnect solenoid that uses vacuum to engage. The two small solenoids you see on the firewall engage and disengage the 4wd solenoid. If you follow the vacuum lines down to the engine they are known to rot and breAk in this area. Sometimes the solenoid on the axle breaks also from rocks. On a rare occasion the fork that engages 4wd can break and you need a new shift fork assembly.
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