Welcome to TRS
The electric transfer case uses a rotary shift rod that turns 270deg from 2WD to 4Low
Each 90deg is a shift position, counter clockwise from 2WD:
2WD-----4High-----Neutral------4low
Then it must go clockwise back to 2WD
There are 2 "Power wires" on the electric shift motor, it is a DC motor so you simply reverse polarity to get clockwise or counter clockwise rotation
The OTHER 5 wires on the motor are to detect the position of the shift rod, and that's the trick, otherwise you are just guessing where to stop for 2WD, 4high, neutral and 4low.
Running motor to either end would cause it to stop but motor would overheat each time depending on how long it kept getting power after it reached end limit of shift rod.
The 4x4 shift module uses the 5 wires to "know" where the shift rod was and where it needed to move(driver selection) and when it got there.
The dash lights just confirm the shift rod's location via those 5 wires.
You might be able to use dash lights and manual control of shift motor polarity to get it into correct position, but is kinda like reinventing the wheel since Ford already did that part.
You don't need the PCM just the 4x4 module, you would need to "trick it" to show transmission in Neutral, brakes on, for shift to 4low
Good look here at the shift motor, drive and shift contact plate(5 wires):
http://www.explorerforum.com/forums/index.php?threads/how-to-rebuild-4x4-shift-motor.123542/
You can get a Shiftster, it bolts onto the transfer case in place of the electric motor.
It allows manual shifting of transfer case with detents that hold it in place.
If you have manual hubs then you can leave transfer case in 4high and just lock and unlock the hubs for 2WD or 4high
You would have to get under vehicle for shift to 4 low