I ha e a 99 siverado that is staying in 4 wheel drive. The truck has the shifter in the floor for the 4x4 but when I place it in two wheel drive the 4 wheel drive light is still on. The truck will only pull with the rear tires like this but the front hubs are still locked in. I pulled the actuator out and tested it when u pull it into two wheel drive the actuator draws in normally and the light goes out for about three seconds then the actuator pushes back out and the light comes back on. What can cause this ?