I have kinda the same issue but mine the low didnt work and now the high is gone as well. Is the vacume box behind the passanger side batt? I now have a funny noise kinda like a buzzing noise that comes from it. Sometimes when I try to enguage it the AC dosent blow out of the vents but the defrost section.
Here's a quick rundown on how the system works.
The switch on the dash controls the electric transfer case motor and a vacuum hublock solenoid that controls the vacuum operated hubs.
Starting in 4x2 position, the transfer case is disengaged, and the hubs are unlocked.
When you move the switch to 4 HIGH, the relays start the motor on the t-case and shift it, stopping when a switch on the motor indicates it's in the right position. The "4x4" light on the dash turns on at that point. At the same time, a voltage signal is sent to the vacuum hublock solenoid for a certain pulse length, which applies vacuum to the hubs to lock them.
When you move the switch to 4 LOW position, the relays start the motor (moving in the same direction) to shift the t-case to low range. Again, the position switch stops the motor when it reaches the proper spot and the "LOW RANGE" light comes on. Nothing happens with the hublock solenoid, as the hubs should already be locked.
Moving back to 4 HIGH, the relays start the motor in the opposite direction and the t-case moves back to 4 high position, and the "LOW RANGE" light goes off. Again nothing happens with the solenoid, because the hubs need to stay locked.
Moving the switch to 2WD, the relays start the motor, again in the reverse direction, and the t-case moves back to 2WD position, the position switch on the motor stops it and turns off the "4X4" light. At the same time, the solenoid is signaled to send a vacuum pulse (either shorter or longer pulse than locking - kind of like a ball point pen button) to the hubs to unlock them.
The OP's saying that 4 HIGH doesn't work and 4 LOW does is either incorrect or something is really messed up in the controls, as the hubs stay locked between 4 HIGH and 4 LOW. Maybe he was just seeing the "LOW RANGE" light and mistook being in 2 LOW (hubs unlocked) for actually being in 4 WD.
On your specific problem, the buzzing behind the passenger side battery is the vacuum pump trying to keep up with a vacuum leak. No vacuum will default the HVAC to blow out the defrost vents. So you need to track down where there might be a vacuum leak. Start by disconnecting and plugging the lines coming from the pump and heading either to the cab or toward the hublock solenoid. If the pump stops then, the line you plugged has a leak downstream from where you disconnected it. Common spots are the vacuum lines to the hubs, the hublock solenoid itself, or someone had stuck their feet under the glove box and pulled a vacuum line loose there. Since your AC goes to defrost when you try to shift into 4x4, the leak is probably on the hublock tubing circuit.