Shakin' while Brakin'

I have a 2001 e-150. About a year ago I replaced the front rotors and brake pads. About a couple of months after I replaced them, I noticed the brake pedal and steering wheel would pulse slightly when braking from highway speeds, onto an off ramp for example. It has steadily gotten worse and yesterday I was braking from 70 mph to exit the freeway and the whole van was shaking so bad I thought is was going to fall apart. Any ideas what to look for before I start breaking it down? Is this possibly a wheel bearing problem or more likely a front end component issue like ball joints, tie rods etc?
Thanks in advance for an advice.

if it starts shaking when the brake is applied this is normally a brake rotor/drum out of round
front rotor = shake in steering wheel
rear drum/rotor = shake in seat or pedal