We did this when we were stuck in 4-lo tinkering underneath in a parking lot, but I assume it would work for you:
Manual t-case operation
Here's a quick remedy until you can find out and repair the real problem:
On the back of the t-case, start by removing the 4 10mm bolts from the actuator and remove actuator from truck. Chock wheels, start truck, engage parking brake, put truck into neutral.
Then using a vice grip, clamp on the "A" shaped pin and turn CLOCKWISE to go from 4-low to 4-high. COUNTERCLOCKWISE to go from 4-high to 4-low. You'll hear it engage just like it normally would.
Turn off truck.
Now the "A" pin doesn't line up with the actuator so install actuator on pin and rotate actuator into position and bolt back into place.
Restart truck.
Your dash lights should indicate you're in the correct gear now. If not, remove actuator and throw in the back seat.
Probably should get it serviced tho, obviously something is wrong.