I am having a problem with my 1998 F150 4x4. The 4WD is not engaging. It is a manual shift. I have checked the transfer case by turning the drive shaft in 2WD it is free. When I shift into 4WD it is does not spin freely. So I believe that is OK. I have checked the vaccum diaphram. It will move the piston when I attach a hose and pull vacuum on the inboard hose barb. So I believe the diaphram it self is OK. I am not sure if the hoses and selenoids are OK. They look fine. However there is a constant vacuum on the inboard hose line (red/pink color) wether I am in 4WD or not. Is this OK? How can I check the selenoids. If all this is OK, the next thing I believ it could be is the shift fork in the differential. Is that a difficult job? Do you need to pull the whole axle out to replace it?

Thanks for your help.