When I'm driving at 55 or less I go to step on the brakes and the rear of the truck shakes badly
My car has 120 miles.
My car has a manual transmission.
This is an indication of a warped brake rotor. When brake rotors get hot and cold repeatedly they can develop high and low spots which causes them to warp slightly. When this happens there is no longer a smooth surface for the brake pads to press against when you apply the brakes. When a rotor is warped and you apply the brakes, depending on how severe the warped rotor is, it can cause shaking just as you described. I would suggest having an expert from YourMechanic come to your home to diagnose your brake rotor issue and make necessary repairs.
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