If the ABS light is on, check the ABS system. From what you're describing with the uneven braking, one of the individual wheel sensors may be sick. Brake pulsing may be caused by warped rotors. Rotors can get warped from over-torqueing the wheel lugs. Who has changed the tires on the car? Unless over-torqued, rotors should not go bad quickly. They usually either fail because of too many rotor turnings (where they machine the faces to ensure a flat surface, and the rotors end up too thin), or warpage such as from too much heat or overtorqueing. Why do they want to replace all four? If they've recently been replaced, they shouldn't all go bad unless they were all over-torqued. Also, didn't they replace the pads when they replaced the brakes rotors the first time? Something doesn't add up. I will admit, dealerships don't like to own up to their mistakes. On a previous Grand Prix (sorry), the rear calipers had to be replaced four times in 30,000 miles. Every time, the dealership tried to tell me it was the fault of my wife's driving style. Every time, some tenacity and logic forced them to reverse course. Kinda funny, now that I've sold the car, GM got socked with a class action on the poor rear caliper design.
