vibration on expedition

I know this is not a tech section or mustang related but I have a ford expedition and the problem Im having is that anytime I hit my brakes when I'm over 50mph there is a slight vibration.But when I hit my brakes anytime under 50mph there is no vibration.My cobra did that when the rotors were pretty warped but it would do it at any speed the rotors on the expedition are a year old the only other thing that I would think off is the brake fluid its pretty dark .Any suggestions are welcomed since I've been trying to fix this for the past two weeks.
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