when on the freeway and I hit the brakes my cars front end starts vibrating
brake system on 2002 Lexus ES300
It is most likely that you have warped front rotors. This is very common on V6 equipped Toyota and Lexus cars. Best thing to do is to replace them after you have a complete brake inspection to make sure that there are no other problems. See http://repairal.com and use the Estimate section for a good idea of what it will cost to replace your front rotors.
It sounds like you have warped brake rotors, have brakes checked maybe cut rotors(if still in specs) or replace them.
Also as trued rotors are being replaced be sure to have the techs torque to specs with a torque bar or torque wrench. this will help ensure that as the rotors heat up and cool down that they stay uniform. Too much torque on one stud and not enough on another can make your rotors get out of round especially if you run through puddles of water after they have already gotten hot. Impact guns are O.K. if torque bar is used.