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Reading your inital post leads me to feel you have no idea how the system works and the service info you need to work with it.
Basicly and assuming you have a switch on the dash, the switch is the first signal the GEM recieves to begin the requested action as well as the switch signalling relays and transfer case motor and vacuum solenoids.
The relays and tranfer case motor return signals to the Gem that set's the mode lamp and operates the vacuum controller on the axle.
Barring water leaking onto the GEM from a bad windshild seal and causing your problem, you have quite a bit of troubleshooting to do.
Replacing the GEM need carefull consideration as it is a programmable module and needs to have the programming in the replacement as it was in the original.
Good luck.
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