Why does the defroster vents only work no matter what setting the selector is set to?

Sounds like you mode door actuator needs replaced.

I believe your model has three ac/heater door actuators . 1) is for mode door 2) is for blend door 3) is for recirculate door

http://www.heatertreater.net/videos.html#chevy --- check this out it may help. There are also several videos on you tube.

Where do I find the mode door actuators?