Shaking, sounds like rattleing when accelerating

Hey everybody, I have a 1996 buick park avenue ultra with a v6 3.8 supercharger engine that has about 217000 mile on it. My problem is, when I accelerate from a stop the car will vibrate. It also vibrates will I accelerate harder in any gear while driving. But stops and runs fine after I release the pedal. It does not shake in reverse. Only in drive. It feels to be comeing from the passenger side front end. I tried to find the problem by putting the car in drive and had someone hold the brake and give it some gas, it did vibrate, but I could not pin point the vibration. Ive changed, the oil, spark plugs, wires, fuel pump, fuel filter, all three coil packs, and the air filter. The car runs great. Accelerates fine, doesnt wast much fuel, it just vibrates alot under acceleration. If I let off the gas pedal it stops. What could it be?