If the heater works properly, your problem is not vacuum. Now you get to hunt down why is it not working. Look at all of the fuses and make sure the are all OK. If the fuses are OK, them you may have a bad shift motor. This shifts the transfer case electrically when you throw the switch on the dash. This will require you or someone else to dismantle thr transfer case to get at the motor. Good Luck, you will need it.
Dave