I recently purchased a 2022 F-250 Super Duty with a 6.7 diesel. I’m having problems with the front axle not shifting into 4-wheel drive. Twice I’ve selected 4-wheel “High” and the dash indicator says that four wheel-high has engaged. Then when trying to move forward the back wheels spin and the front wheels do not. The last time this happened I shifted from 4-wheel “High” into 4 “Low” and then the front axle engaged. Then when I shifted back to 4 wheel “High” the front axle remained engaged.

The truck was idling each time these shift attempts were made. I understand the front axles use vacuum to engage the system. Can it be that I wasn’t creating enough vacuum to engage the front axle at idle? Also, why would the dash indicate that 4-wheel drive was engaged when it really wasn’t?

Also, traction control was in the "off" position so that could not be the cause of the problem.

 

If there was a way you could see if the front driveshaft is turning when slightly moving the vehicle would verify if shift has completed inside the transfer case.
Depending on if it has or not , we go from there.

 

In my old FWD I've had vacumn lines go bad: get burned, crack, or become disconnected. Additionally reseating the mounting screws on the FWD control module, re-established the ground to the body. An old tech told me 60% of all electronics problems are bad grounds. Good luck.