Violent shaking 2002 Bonneville supercharger

I have a 2002 Bonneville ssei supercharger, when it
coast/accelerate it begins to shake violently when it
hit 40mph. I've change the brakes, rotors, tie rods
and hub bearings. I've also notice that the motor
mount may be worn out or broke could that
possibly be the problem...and I've also notice that
when it does shake and I hit the brakes it slows and
stops smoothly with no bouncing feeling in the
brake pedal and the steering wheel doesn't really
shakes or vibrates...any and all suggestion
welcome