Just to clarify, you do have the manual transfer case shift lever on the floor console, correct? If you shift to 4WD LO, does the LO range actually engage? Keep in mind that with the front driveshaft turning freely, only the rear wheels will be powered. This will determine if the transfer case is able to shift the range from HI to LO.
With the engine at idle: If you can apply and release the vacuum at the actuator by moving the shifter from 2WD to 4WD and back to 2WD, it means the linkage inside the transfer case is at least operating the vacuum switch and all of the vacuum lines are good.