If the vehicle has a lever for engaging the transfer case, there is no encoder motor. The motors are used on vehicles with a push-button transfer case select switch. The motor basically does what the manual lever does in moving the transfer case linkage.
Both type systems used a vacuum switch (valve) at the transfer case, however. When 4WD is selected the valve is actuated and ports vacuum to the front axle actuator under the battery tray. Of course, vacuum must be present at the switch/valve in order for that to happen. It is not uncommon for the vacuum line to the transfer case to deteriorate. There is also a vacuum reservoir bolted to the underside of the hood on the driver side, and it has to have vacuum as well, since that serves the transfer case as well as the HVAC controls and cruise control.