1988 v10 4wd actuator help!!

so basically i am helping my father repair his 4wd on his 1988 suburban. when we shifted the 4hi-4lo-2hi lever the transfer case engaged, but the hubs did not lock, and they are not manual locking hubs, or so i don't think, there is nothing to twist on the hubs them self. but we went to our local auto parts store and bought and electric 2 prong actuator (the electric actuator that heats up to move a pin and engage the hubs.) but i can not locate this anywhere on the front axle/diff, nor the tcase. and i was notified that it is vacuum controlled. but i can not locate this either and am having a lot of problems dealing with this. i can not find anything online on where it might be located or even how to do it. i have found videos on how to replace bother electric actuators and vacuum diaphragms. but not for his 88. so please any help is appreciated.