4 Wheel Drive/SUVs: 4 wheel drive, drive shaft, test light

Question
I just purchased a 1996 Z71 to my surprize when i engage to 4 wheel my hubs are not locking front drive shaft works good any idea why? It is an auto trans. with manual 4 wheel drive not push button.  

Answer
Troy, sorry for the delay , been busy fixing 4x4 problems in the shop on chevys. lol Any way take a test light to the front actuator and see if you have juice going to it or not. That is 99% of the time what is wrong with these trucks , let me know if you need anything else, thanks ADAM