Re: 98 f150 front diff not kicking in
The truck probably has the "automatic" hubs if it has a dash-mounted switch for 4WD. There should be 2 positions on the front hubs, marked "auto" and "locked" or something similar.
The purpose of the hubs is to connect the axle shaft to the wheel when in 4WD and disconnect them when in 2WD. They are actuated by vacuum, but can be locked manually if the vacuum system fails.
If the 4WD is engaged by pulling a lever down on the floor, then the truck has manual hubs, marked "free" and "lock" or something similar. These must be locked or unlocked manually.