I'm hoping for better fuel mileage
can I disconnect the AWD to make it a 2WD then I can switch to 4WD whenever. on 1999 Ford Expedition
No, you cannot. You should notice when not locked into 4WD your truck should make tight turns easily and not feel like its binding up. In 4WD it will take some throttle to make it move on tight turns, that difference would show that it is already disengaged the way that it is designed to be. The transfer case has a differential assembly built into it that makes it a 2WD when the AWD is disengaged. But if you pull the driveshaft to try and reduce the drag that it still has the transfer case can be damaged.
Yeah your screwed,keep paying/that gas bill,can't have the best of both worlds yet..