All of the vents on the dash are blowing warm air, but all of the rear vents are ice cold.

What is going on?

 

Sounds like your H valve may have gone bad or clogged. The H valve controls the flow of freon to the front and back of your A/C.

You may have to take this to a A/C shop and have your system evacuated as most common problems to the H valve not working right is contaminants in the freon. Otherwise, it may need to be replaced.