You should check belt tension and visually inspect the pulleys for rubber deposits. You can check pulleys by applying a load to the pulley and spinning it.
You probably need to include which transfer case you have on the vehicle by looking at the stamped plate.
How much vacuum is "little vacuum"?
If you have a NVG231 there should be engine vacuum (roughly 20inHG) at the shift motor. If there isn't you need to check the lines to the vacuum switch and vacuum before/after the switch. Though the way the system is described lack of vacuum to the shift motor would be defaulting to 4WD. When 2WD is selected the valve opens and the motor unlocks the right front axle.
You mention lights do you mean a dash light to indicate 4WD or an electronic switch with 2WD, 4 Hi and 4 Lo?
If you have an electronic switch you may have the NVG243. In which case you should check fuses first. If fuses are OK you're going to need a scan tool capable of bi directional controls to test the transfer case and front axle locking motor.