If you do a search on this, you will find many threads that deal with this issue. It is most likely what I refer to as a 'brake shake' caused by a deformed or warped brake rotor. Since you don't mention what year 4Runner you have, all I can say is to have your rotors/brakes checked. If you have a 4th gen, then you will have 4 wheel disc brakes, and it could be either the front or rear rotors causing the problem. The initial problem is quite often caused by the rotors getting overheated from heavy braking use, a frozen piston in one of the calipers, or in some cases I have seen a 'hot' rotor warp just after driving through an icy puddle. The sudden temp change to the metal when the cold water splashes on the hot rotor has been known to cause it to warp suddenly. Most of the time, the problem starts out with a slight steering wheel shake when braking, and as the problem gets worse, you might feel a 'surging' pulsation just as you come to a stop. As others have said, the only fix is to get the rotors resurfaced (which quite often is only a temporary fix), or replace them (which is my prefered suggestion), which considering the labor costs to properly resurface a rotor, is probably not much more cost wise.
