1997 to 2004 heritage use a center disconnect front differential. The wheel hubs are splined to the front halfshafts and are locked all the time. Like a front wheel drive car. The front differential uses a splined locking collar that slides over the axle shaft in on the passenger side to lock that side of the differential. The driver side is one piece. When you engage 4x4 a vacuum motor pushes a fork that moves this locking collar over the splines to lock the front axle. Common failures are the shift fork wearing out. The vacuum motor failing. The vacuum lines splitting and leaking. Finally a defective actuator solenoid.
2004 + use an IWE ( Integrated Wheel End ) system. A spring pushes a locking collar over the splines in the front halfshaft to lock the two parts of the halfshafts together. The front differential is permenantly engaged as the IWE eliminates the need to engage / disengage the front axle. The default position for the IWE is engaged. Vacuum us applied to a diaphragm that pulls the collar off the splines. In the event of a vaccuum failure you will still have a locked front hub allowing you to drive out of the muck. Ealier modles of this system had problems allowing water intrusion into the vacuum system. The water could cause problems with the IWE and or the actuator solenoids on the firewall. Common failures are the water problems and or defective solenoids or the IWE itself. As the default mode is engaged, a vacuum leak can cause the IWE to engage or worse partially engage. That causes the splines to grind and destroy the IWE.
I found the link that explains Ford's IWE