When accelerating, only when foot is on gas pedal whole car shakes violently ???

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only when accelerating, my saab will begin to shake violently. Just bought the car yesterday, doesn't matter what gear I'm in, shaking still occurs. When I let off the gas, the shaking stops. Tires are fine, previous owner told me its a CV axle, but I've also read that's probably not the case... Please help!