My first guess would also be warped rotors. Especially if you do not hear any grinding that would indicate worn out brake pads.

Believe it or not, warping can be caused by over-tightened lug nuts. Have any of your rims been removed lately? Perhaps during a recent safety inspection? I would supect that the inspection guys do not re-install wheel nuts ( bolts in the Crossfire's case ) at the proper torque. They generally use an air wrench and make it hard to hand loosen whenever you need to get the wheel off yourself.

Since I haven't seen others reporting rotor warping problems, this does not seem to be a general Crossfire issue, such as seat heater problems.

Also, with warped rotors, the vibration usually increases with speed. You should feel it under all braking condions and not just extreme conditions.