Why would a 2000 Subaru Legacy Outback Wagon steering wheel and front end vibrate and shake when you hit the brakes while going down a hill in the mountains?

Sounds like your brake rotor are warped, a brake shop should be able to turn then (smooth them out) for you if there not too bad. do you feel a little shake at during normal breaking? if so then that could be the problem, but because your were going down hill it take more force to slow down so you feel it more.