Q: 4wd wont engage

ok i have had jay safford helping me and hes doing great but i am getting mixed answers. so i am still asking about my father 1988 v10 suburban, and so how do the auto locking hubs work? how do they engage and disengage? what can i do to fix them if there is no sort of actuator or vacuum actuator for the hubs? cause once again when shift into 4hi or even 4 lo the hubs wont lock. and i am trying to figure out what i need to replace in order to get the hubs to work. if the case is i do not have these actuators. then i can take apart the hubs and re grease them and clean them or get new hubs, but is there any other sort of mechanism that engages them? how do they engage? please give me a little more info than just this and that please be helpful.

My car has 100000 miles.
My car has an automatic transmission.