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When you say have power to the front wheels does that mean the transfer case is working ( turning the front axle but the hubs won't engage?) When you say it is getting vacuum to the pump do you mean the solenoid for the vacuum hubs?
After you bypassed the solenoid did you check for vacuum at the wheel end vacuum connectors?
Ok If you bypass the solenoid and the hubs don't engage then it sounds like there is 2 problems. One there should always be vacuum at the source side of the solenoid and the other port should have vacuum for approx 60 seconds after the 4x4 switch is engaged. This is how they work:
When the GEM verifies the completion of the transfer case shift from 2WD to 4X4 HIGH (reading the contact plate positions), the GEM will then energize both pulse vacuum hublock (PVH) solenoids (supplying ground for 45 seconds). The PVH solenoid will allow a HIGH flow of intake manifold vacuum to the "wheel end hublocks", engaging the wheel end hubs to the front differential and transfer case to achieve four wheel drive.
When the GEM verifies the completion of the transfer case shift from 4X4 HIGH to 2WD (reading the contact plate positions), the GEM will then energize only one (low vacuum solenoid) of the PVH solenoids (supplying a ground for 10 seconds). The PVH solenoid will allow a "LOW" flow of intake manifold vacuum to the wheel end vacuum hublocks, disengaging the wheel end hubs from the front differential and transfer case to achieve two wheel drive.
You need to verify the gem module is grounding the solenoid properly and that it has power
The second problem of the hubs not engaging when you bypass the solenoid has to be a leak in the lines, wheel end o rings or hublocks leaking. You would need a hand vacuum pump to check for vacuum bleed down in these components.You can also bubble test the system with soap and 2 psi air pressure in the lines.Do not exceed 2 psi or you can ruin the hublocks. A spray can of compressed air will work if you limit the pressure.