Re: F250 4 wheel drive operations
What flavor of 4WD system do you have on your truck?
If, for instance it's the manual system, you may have a hub that's not engaging... If you have a manual transfer case with the automatic hubs, it's also a good possibility.
If you have one of the "push button" 4WD systems (I'm not sure what was available in '99) then it could be either the transfer case failing to engage the front prop shaft or a hub (if it has hubs - some of the "push button" systems leave the half shafts coupled all the time and just disengage the prop shaft).
If you understand how your particular system works (or is supposed to work), it's very simple to diagnose with the truck on jackstands.