Im sure its been suggested, but follow the 4x4 module self test procedure in the tech library for that model truck. Its mounted in the body behind the jumpseat. Then try the shift motor cleaning article. The shifter rotates one way for 4hi then even farther for 4 lo. If something is dirty r blocking the shifter from fully rotating it was go back to the point where it started. A
As far as shifting procedures, like everyone said 4hi can be accessed anyway at all below 20mph, (mine actually kicks in at any speed) But 4 Lo you must be
a. complete stop
b. clutch pushed in
c. neutral
d. (alot of people dont realize this) but little to no stress on the drivetrain. If tires are worn uneven then the front pulls at a different speed thanthe rear, which puts uneven stresses on the drivetrain. The shifter will recognize this as the wheels puling in motion. If you are driving in 4hi then come to a complete stop, reverse about 4 ft, then stop and go thru 4lo procedures, If that dont work, then turn the truck off, turn it back on engage 4hi then 4lo consecutively. This shoudl do it. After the self test and shift motor cleaning.