I'm having problems with my 4x4 trying to engage without any imput from me through the shift lever. I believe the problem may be with the GEM. I'm able to get the lower part of the dash off but after that I'm lost. There seems to be another plastic case enclosing the fuse panel and GEM. Does anyone know how to access this. I have read on this forum that the fuse panel has to drop down before you can get to the GEM. Any help would be appreciated.

 

The pre 97's have to come through the engine side of the firewall. Donno 'bout the newer trucks.

 

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.