I have a 99 F150 with the electronic shift system and it will not shift into FWD, it will shift into low but only 2WD

 

to shift into 4wd LOW you need to apply the brake and put it into neutral before switching it into 4wd low.4wd high you should be able to switch into anytime and the 4x4 light will light up on your dash,hope this helps.

 

There have been many postings about the vacuum shift motor and the vacuum line which runs to it in the Drivetrain forum.