What could be causing a terrible vibration when braking (especially heavy braking as done exiting the highway). Its a 1998 f150 4x4 xl.
Parts changed so far:
rotors
pads
calipers
master cylinder

 

If the only time you get a vibration is when you’re applying the brakes it could be warped rotors.

I see you have changed your rotors, but vibration during braking is a sign they could be warped.

 

Depending on where you buy them from, new rotors can be a little un-true. If they're some no name jobbers, I'll put a light cut on them, taking off about .003-.005 on each side.

When you bolted the new rotors on, did you clean the hubs best you could? Take a wire brush or, if you got one, a die grinder to the hub faces and clean all the rust and crud off them you can before sticking the new rotor on.

While driving, try slowing down using the parking brake only to eliminate the front brakes as the source of your vibration as well. My '97 F250 LD shakes like hell when pulling down from highway speeds, I gotta resurface the rear drums or replace 'em someday, I'm just too lazy to do it right now. :o