1989 Chevy Silverado

Hello my truck is making a clicking soung coming from the front end when I drive down the road the 4wheel drive wont engage. The lights on the 4wheel drive on the floor say it is on but it isnt powering the front wheels is this a hub problem? It has automatic locking hubs on it and I called my local autozone to price hubs and they said that they couldnt pull any up for my truck they found some for a suburban though? Im confused as to how my truck cant have hubs on it.