1994 Chevy Silverado 4 wheel drive

My truck has the 4-wheel drive shift selector in the floor. It is a manual selector, not a switch. When I select 4-wheel high or 4-wheel low it feels like it engages. What I am basing this on is when it is in 4-wheel low the gear ratio drops. My question is: Is there an electric switch in the system that activates the 4-wheel drive much like locking hubs. I do not have locking hubs. There is no noise or grinding but the front wheels do not activate. Nor does the indicator light show that they do. I have checked the bulb, the front wheels just do not pull. I am thinking that there is an electrical switch in the system somewhere that activates both the light and the front axle
Look forward to your response.