^ What speed is it the most pronounced? I would think if it's an tire or balancing issue it would be more pronounced at high speeds. If it as an anti lock problem you would almost be certain to have a fault light. I still wouldn't rule out the rotors. It's possible one or both of the rotors are slightly out of round. You may not in this situation have the traditional vibration-pulsating in the pedal you come to expect in a warped rotor, however; in a heavy braking situation the vibration you are describing seems to possibly indicate an out of round rotor condition.