My dad has an '05 and the other day he tried to use the 4wd all he got was a ratcheting sound from the front. He told me about it so I
checked it out and found the following:

With the engine off the IWE's are locked. With the engine running the IWE's have vacuum going to them in both 2wd and 4wd. I pulled off the vacuum line going into the IWE and it instantly locked in. I could still feel suction in the vacuum line with the transfer case in 4wd. Any ideas what is wrong,
I'm guessing a check valve or solenoid?

 

https://www.ford-trucks.com/forums/8...then-some.html

 

my brother had the same problem check your wheel bearing his was bad and the gear on the bearing was in a angle rubbing on the gear on the actuator and worn the gear.
they don't sell the gear alone so you may need to buy the actuator just for the gear alone and keep the actuator till your old one is damage
look the the second locker half the gear is gone.
this will make the ratchet sound till it cant be warn any further.
usually the valve stop pulling. making the actuator being lock full time

 

Got the hubs to engage by replacing the solenoid on the firewall but I think he also has a slight vacuum loss somewhere because when the engine is under a load I can hear a really faint tick coming from the drivers side. I pulled the vacuum lines from the IWEs and the sound went away so I'm guessing it is 4wd related.

 

I believe the hubs are ENGAGED normally and it takes VACUUM to pull them out.. If you remove the vacuum, the front hubs should be locked in 4 x 4.... but your transfer case would not be engaged.

 

you can also bypass the valve to have it unlock fully i can send a picture to show you also if you try this if it still doesnt unlock them you have a leak.
i had some debris going in them that would stop them from locking to 4x4 too..

 

i took the lines off and let the truck blow them, there wasn't much pressure so I took the the check valve and tapped it on the fender now there is more pressure guy i have to hook uo to boat to see if the thing is fixed..thi only happen when pulling boat or camper quits when you let off the gas.

 

you have a misunderstanding of how it works... The 4 x 4 is ENGAGED when the motor is off.. You have to have the motor running and the solenoids OPEN on the firewall to have VACUUM on the hose to each IWE.. The VACUUM pulls the hub back into 2 x 4.... When you let off the gas you have more vacuum... When you push down on the pedal you have LESS vacuum... either you have a vacuum leak, or things are sticking. Go back to the original link to "how to test and repair" the IWE.