I had my brakes and rotors replaced over a year ago (too bad the warranty is up now) and every time a go down a really steep hill (but only on hills, not flat) and hit the brakes, whether hard or lightly, the brakes make my whole car vibrate in a chugging motion, as in, not a high frequency vibration, but more slow and heavy. Any ideas?

Warped rotor?

Warped rotor?
Probably. It's no big deal to get the rotors machined, and shouldn't be too expensive.