ENGINE VIBRATIONS ON STARTUP AND DURING DRIVING. WHAT COULD BE THE CAUSE?

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MECHANIC CHANGED 2 MOTOR MOUNTS AND TRANS. MOUNT, CHANGED STRUTS. CAR SEEMS TO VIBRATE MORE SINCE THEN. STILL NEED TO CHANGE MOUNT ON PASSENGER SIDE. CONSTANT VIBRATION FROM WHEN I STARTUP AND INCREASES INTENSITY AS I ACCELERATE AND MINIMIZES ON DECELERATION BUT ALWAYS CONSTANT. AUTOZONE FREE DIAGNOSIS SUGGESTED BAD CATALYTIC CONVERTER FRIEND SUGGESTED BAD INJECTORS AND RUN GAS TREATMENT. ASKED A ASE CERT. MECHANIC AT FIRESTONE, HE COULD NOT FIGURE IT OUT. TOLD ME I JUST HAVE TO DEAL WITH IT. I CAN FEEL THE VIBRATION TROUGH THE GAS PEDAL AS I DRIVE. HAVE DRIVEN OVER 3000 MILES SINCE PURCHASING USED FROM A DEALER WITH 159000 MILES. RUNS GOOD, JUST HAS THAT VIBRATION. HAVE CHANGED SPARK PLUGS, WIRES AND CAP AND ROTOR AND AIR FILTER, OIL AND FILTER, TRANSMISSION FLUID WHICH WAS REALLY DARK ACCORDING TO MECHANIC AND FUEL FILTER. HAS THIS HAPPENED BEFORE ON THESE CARS. I HAVE THE BASE MODEL WITH AUTO TRAN. ANY INFO WILL HELP ME THANKS 2000G20