Problem is most likely Neutral switch/sensor, can't shift into 4low unless system "sees" that you are in Neutral, and...............speedo is at 0mph, and foot is on the brake pedal usually
Neutral in a Manual trans is clutch pedal down
So did you wire up the Clutch switch?
On the automatic's DTR sensor was a Grey/purple stripe wire, it runs to pin 13 on the 4x4CM(control module)
This wire needs to be Grounded to indicate Neutral to the 4x4CM
Unfortunately Ford must have bought alot of the grey/purple wire, lol, because there are 3 of them on the DTR sensor
Pin 8 on the DTR has the grey/purple wire you want, ground it
If you wired in the clutch switch then you could use it for 4Low
Clutch switch has 3 separate switches inside
All are OPEN when pedal is UP
All are CLOSED when pedal is down
One switch is used for activating starter motor, another is used for REV limiter, 3rd could be for 4WD system
Connect grey/purple wire and a ground wire to that one switches contacts, clutch pedal down = Neutral