My steering wheel has begun to shake on my mazda 3 when I am brake while going over 50 mph. This leads me to believe that I have warped brake rotors at only 18000 miles... However one of the common symptoms of the brake pedal pulsating isn't happening, only the steering wheel is affected. Does anyone have any other ideas what this might be? If it is the brake rotors, how much does it usually cost to have them trued?