The 4WD system equipped on this line of truck is somewhat actually genius. They feature a knob style switch that locks the hubs and shifts the transfer case. The shifts can safety be performed on the fly at up to 55 MPH. The switch has three options - 2WD, 4WD Hi, and 4WD Low. Once the switch is moved, the computer makes the shift happen. The transfer case is shifted with a electronically controlled motor.
The axles are locked by a vacuum operated sliding gear sleeve that mechanically couples the driving axle to the hub and bearing assembly. The odd thing about this system is that it uses vacuum to force the truck into 2WD mode. Once vacuum is cut off, the hubs will lock the front axles. It actually creates a fail safe in the system because they opted to not offer any sort of manual lock outs on these trucks. If the system fails it generally locks the front axles. The common failure points are the actuators (geared sleeve), vacuum lines, and solenoid. A common symptom of a failure of a vacuum leak is a loud metallic growling noise coming from a wheel during low engine vacuum events such as moderate to severe acceleration or maybe even all the time during 2 wheel drive. If you only get a metallic growling noise when in 4WD, it generally means a vacuum leak has damaged the actuator bad enough by partial application that it will no longer correctly spline to the hub. The actuator is aluminum and by design should fail without much damage to the steel hub.
I recommend you check out the Dorman products. Dorman makes a good inexpensive replacement 4WD Hub Actuator as well as a redesigned 4WD Solenoid that features a hood and sets the solenoid upside down to deter water intrusion.
If you move the switch into a 4WD mode and 4WD light does not illuminate, you have an electrical issue with either the solenoid or the transfer case. If you suspect you have an issue with the front lockouts, they are easily tested with a vacuum pump. Performing all diagnostic steps (in any order) is critical to the proper diagnosis of this system. Depending on the situation, a failed actuator is sometimes just a SYMPTOM not the CAUSE of the concern. Example: Low vacuum to the actuator over time has damaged the actuator and replacement of just the actuator will cause it to fail again.
To test the system, you will want to raise the front tires off the ground with the vehicle in 2WD with the ignition in the off position. Be sure to follow all safety precautions and use a jack stand. With the tires jacked up off the ground you then should spin a wheel. It should be already locked in or you should feel it lock in after turning it. You can verify it is locked by watching the constant velocity joint (the front axle) to make sure it turns with the wheel. If it does not, you will need to replace the actuator for failing to lock. After replacement or passed testing, continue on.
If it passes that test, start the truck up. Go back and turn the wheel after it has ran for about 10 seconds. If it unlocks from the axle immediately or after a few turns (spin the wheel and front axle does not turn) then proceed to use the vacuum tester. Remove the vacuum hose from the back of the actuator assembly and attach it to your vacuum gauge. You should be seeing close to 20 inHG of vacuum with the engine idling. If you see less than 20 inHG but more than 5inHG then you likely have a leak either in any of the vacuum lines or the other actuator. If you see at or close to 0 inHG then either a vacuum line is completely severed or the solenoid is not working properly. Then connect the vacuum pump to the actuator and apply a vacuum of near 20 inHG. The vacuum should not bleed off more than a few inHG within 30 seconds and if it does, replace the actuator.
Reattach the vehicle vacuum line to the actuator assembly and repeat these same tests on the other wheel. If at any time you notice water or rust in the vacuum lines, try to drain the lines and replace the solenoid. If the actuators are working properly and receiving the correct vacuum, try putting the vehicle into 4WD and testing the available vacuum at the actuator. The vacuum should bleed off and rest at zero. If it does not, it is likely that the solenoid is stuck in the on position and always allowing vacuum to the actuators.
Check out my other article for step by step directions, required tools, and pictures on 4x4 Actuator Replacement on 2004 and up F150.