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If it's not switching ranges, then there's a problem somewhere in the transfer case motor setup. Could be a bad relay block or just corroded connectors on the shift motor harness too. The motor could be bad too. If it's intermittent, I'd suspect a bad relay block or connection first.
You can test the motor itself by unbolting it from the transfer case and watching to see if it rotates clockwise/counterclockwise as you try to select a different range. If it is turning, then unfortunately it probably means that the problem is inside the transfer case.
You can also try to turn the shifting cam shaft that the motor attaches to by hand when the motor is removed...there is nothing to hold it in place though so this is just a shop test procedure. Don't take off down the road with no motor attached to the transfer case...it can jump into different ranges on ya. The case has marking on it that will index with the shaft to let you know what range the transfer case is in. If the transfer case can be shifted manually, then you'll need to look into the motor power setup some more.
I'd still check out the front axle vacuum actuator lines, and solenoids as jbrew suggested too but the GEM module is dumb. (There's no feedback from the actuator to tell the GEM if it's engaged or disengaged or if you've got broken vacuum lines...it'll say it's shifting but just won't operate the front axle disconnect)
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