GMC Sonoma (1999) 4-wheel drive problem

Can anyone give me a brief synopsis of how the 4-wheel drive works in a
1999 GMC Sonoma? Are the front axles engaged all the time? Are there
hubs to be locked, or automatic? Is there something (solenoid) that
engages the front differential to the axles? Is this operated by electric
or vacuum? Thanks

there is a vacuum pot with a cable to the front axle located under the
battery. when you put it in 4 wheel drive, a solenoid and vacuum switch on
the transfer case engage the front driveshaft and then the vacuum pot under
the battery engages the front axle. there is also a solenoid--on or near
the firewall, that supplies vacuum to this pot when the dash buttons are
pressed. If you have the automatic 4x4, and its not working, the plug on
the controller is probably rotted out... check for it under the right
front kick panel inside the cab.