Car Vibrates when going 50MPH

[89FHPLX]

OK this just started, When I'm in 5th gear in the highway and I let go of the pedal the car vibrates(HOPE MY SPELLING IS CORRECT). Even in fourth when I start to gun it a little, you can feel the shifter and the floor, and well basically the whole car shake. Tires are brand new, blance, and botls are tight. I recently heard tonight a noise in my rear brakes like if something was loose in there. but the shakeing of the car is really bad. its worse when I give it gas and when I let go of the gas going 50mph or more. Can this be a Mount? I hope you guys have a Idea what this can be. My car has 145,658 miles on it but its always keep in very good running condition.

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89 Police Interceptor
BBK Shorty Headers, K@N Cone Filter,H-pipe no cats, 2 chamber FlowMasters,Under Drive Pullys,14 degree Timing, Short Belt, No Air Silencer, oil cooler, 3.08's, And All Police Package goodies