Loud bang from the rear end, car wont move? Details in the question..?

I was going about 25 mph, cause it was a school zone... there was a slight incline, and i heard my transmission downshift, then a very loud BANG. My low trac light came on, but i know for a fact that i do not have traction control on my car. I pulled over put it in park. Well, waiting for the tow truck, i decided to see if it would move. I put it in gear, and the transmission engaged, so i know its not an issue with the transmission. I was leaning towards the differential (Its a 99 firebird, Rear wheel drive)... but when I gave it gas, i could feel the car lift the front end up, and when i didnt give it gas, i didnt roll back. I was parked on a hill. Could it be a brake line that snapped? and now my brakes are stuck? Or is it for sure my differential?