No. The vacuum switch is its own item. It is the three-pronged plastic piece that all the vacuum hoses come off of. However, the switch does not activate until the transfer case is in 4HI. The transfer case does not shift to 4WD until:

1. Driver selects 4HI on Transfer Case Shift Select buttons (TCSS)
2. The TCSS sends a request to the Transfer Case Control Module (TCCM). The TCCM has input signals from other items like speed sensor, etc. If all the signals are correct:
3. TCCM sends demand to the Encoder Motor.
4. The Encoder Motor shifts the Transfer Case to 4HI.
5. Transfer Case (in 4HI) activates (closes) the Vacuum Switch. When the Vacuum switch is open, it is venting to the atmosphere.
6. Vacuum is sent to the front axle actuator.
7. The actuator pulls the cable which moves the fork to lock in the front axle.
8. When the front axle locks in an electric signal is sent back from the front axle electrical connector to the TCCM which in the end sends signal to light up the 4HI lamp on the TCSS.

Let's back up a little here, what type of transfer case do you have? It's on a metal tag on the transfer case.

If I were in your shoes, I would start with the test for fault codes as listed in the DIY thread https://blazerforum.com/forum/tech-a...ill-help-7180/