My brakes just started vibrating occassionally when stopping on highways at high speeds. I have been realizing that the brakes feel low. I went to a shop for an oil change and the mechanic peeked in between the aluminum wheels & told me that I definitely do NOT need to cut my rotors but I do need to change my brake pads. Why would I be vibrating when braking if there is no need to cut the rotors? and is it possible that I only need to cut the front rotors and not the rear?