Warped rotors cause a vibration in the brake peddle or steering wheel usually at higher speed But maybe it could be warped rotors. Its very hard to fix a car on line. Plus you are not conferrable with working on your own car this makes it even harder on my end. You may just take it in an have it looked at by a good shop.

I am thinking it a suspension because you do not have a problem with it locking one tire before the rest and you do not have a problem with a pull when you hit the brakes.

I do think it is a safety issue and is worth getting it fixed. Ever thing else can fail on your car but you always need your brakes.