Like 7700 says, these trucks have no locking hubs. If the axle pod is good, right behind the battery is the two solenoids that control the vacuum to the front differential. You can check those vacuum lines for cracks or breaks at those ports. Good luck
Born and raised in Alaska, and damn proud of it.
1998 F150 Scab, 4.6, 4R70, 8.8 w 3.55ls, XLT. Power strg, windows, locks, drivers seat. 123k troublefree miles. Lund hardshell tonneau and Lear bed shell. 1990 F150 sb 4x4, 5.0, AT Lariat, 88k miles, tonneau and shell. 2nd owner. 98 F150 FS 4x4 reg cab, 5.4.