Brake light switch?

Due to higher gas prices I have not been driving the Denali as often as I used to. A problem that started to come up the last few weeks I drove it daily was not being able to shift out of park to any gear. I had to pump the brake and try to shift at the same time. I put this on the back burner since I stopped driving it. Well the Chicago winter is here and I was thinking about using it when the roads are bad. The other night I noticed that when pressing the brake pedal, the rear tail lights would not be on. I also noticed that when the tail lights are off that I was unable to shift out of park as well, all of this while pressing the brakes. I am assuming that there is a sensor that sends a signal to the trans to shift and for the lights to turn on. What sensor would this be and how easy/hard is it to fix?

Thanks