Nebulous, we need your axle code off of your truck so we can start looking for which 4x4 conversion parts you'll need.
Ford Motor Company drive axle gear ratio and transmission gear ratio codes may found on the Vehicle Certification label or the Vehicle Rating Label. The Certification label is typically attached to the door latch edge on the driver side door.
This label is from a 1999 Ford Explorer, the axle code is "42", and the transmission code is "D".