Pretty simple (bolt on) if you have the parts (can pull from parts yard). Your truck already has the 4WD suspension.
Parts Needed:
- 4WD Spindles
- 4WD Front Wheel Bearings
- CV Axles
- Front Differential (gearing matched to rear)
- Front and rear driveshaft (rear gets shorter due to transfer case)
- Transfer case
- 4WD transmission (or 2WD trans rebuilt with 4WD output shaft, which is a different length)
- 4WD computer (or other controls)
- 4WD switch
- 4WD ECU if you want everything to play nice (you can use 2WD, but not everything may not be operable, like speedo reading correctly in 4 Lo).
If you keep an eye out on the explorer/ranger forums, sometimes you'll find someone who pulled most of this off to SAS their truck who is willing to let this go for a song.
I'd get electrical parts from the same year/model as your truck, but hard parts can be pulled from a bunch of different trucks. There are some slight differences in parts, but most should work.
Donor trucks to look for:
1998-2011 Ford Ranger/Mazda B-Series
1995-2001 Ford Explorer/Mercury Mountaineer
2001-2003 Ford Explorer Sport
2001-2005 Explorer Sport Trac
You'll need more details than I give here, but it's not rocket science. Ten years ago I priced out the used parts to do this to my Ranger Edge and I want to say parts were going to run $700-1200.
If you're serious about this, head over to explorerforum or one of the ranger forums to get more info.