When is the last time you had your struts done? I've noticed a lot of sagging rears on this forum. Get the rear springs, struts and rear end linkage done.
I just had all of this done at Monroe and I LOVE it. The car was coming to a stop as if it were a spastic whale...lots of vibration. What was happening was my rear weight was being dumped foward when I was braking because the rear struts weren't holding and the front struts couldn't support the extra weight. Thus: vibration and terrible braking.
Also, check the rear springs. Old Ford designs only used 7 coils on their springs. This would cause weak spots and make the springs break at the top or bottom, thus destroying ride height and quality and causing agressive braking issues. New replacement springs have 8 coils to prevent this from happening again.
If it were warped rotors, the car would most likely yank to one side and squeak like ****. Go outside and look at your rear end. Is it sagging?
So yeah...sounds a lot like a suspension problem. And warped rotors DO NOT slap fix themselves. I'm sorry, that is just a ridiculous statement! laugh_ti