Diff not locking in 4wd

[Country Boy]

I have a 99 ford expedition 4x4 and when i switch it to 4h or 4low the axles wont turn like the hubs are not locking in and it does nt have the actuator and i cant find the vacuum switch for the hubs. Can anybody tell me where it is and can that cause the front axles not to lock in when switched to 4wd.

 

[trdragons44]

The only thing i can think of is that the center disconnect on the front axle is not locking. There should be a cover on the passenger side of the differential; under it is the actuator. The solenoids are behind the battery against the firewall; there should be two one for lock the other to unlock the disconnect.