I saw a post some time ago where you could remove a relay and change your Expy to a 2WD/4WD drive system. My question is can this be done in reverse? What relay can I add to my relay box in the engine compartment and make my wifes '01 F150 into an AWD system?

 

Unless the F-150 has the proper AWD transfer case with the ability to differentiate power forward and rear, then it isn't possible.

 

You would need a different transfer case.

Adrianspeeder