99 F150 XL 5.4 9.75 rear 8.8 front

A while back my 4x4 quit working on the front axle. Lights come on as usual and I thought it was working. But the front does not engage. When its in 4low the front drive shaft turns as normal but the tires dont turn? When you take it out of 4low/hi the driveshaft stops like normal. Any suggestions? Havent taken apart the diff cover yet hoping it might be something else besides spider gears or worse. Truck drives fine though, no noise from the front at all. Thanks

 

There is an internal differential lock instead of locking hubs. That's what you need to look at. Most likely the internal shift fork is broken, but there could be a problem with the actuator system.