New to the forum. I have a 1997 f150 4.2 lt., 4x4. The front axle won,t engage in 4 wheel drive. I checked the vacum at the selenoid on the front axle. When I move the shifter from 2 wheel drive to 4 wheel drive the vacum does not change from one hose to the other. Because of that it cannot move the rod in the selenoid back and forth to engage the front end.

 

Check the lines all the way back, if I read what you are saying is that you are not receiving any vacuum at the solenoid. Check for any breaks or cracks.

 

X2 ^^^^

and welcome to the site