I have a craftsman LT2000 with a 21hp single Briggs engine that has me stumped. It has a bad vibration when blades are engaged. It is so bad whole motor shakes. I have changed blades, no change. Run without blades, no change even swapped a perfect running deck off another mower, no change. Have used a straight edge alongside of engine pulley while running. Pulley runs true with no wobble. Motor runs great and without deck engaged it is smooth but when deck engages it shakes real bad. Motor mounting bolts are tight and mounting surface is not cracked anywhere. Motor has no vibration at all while running free but when deck (no matter which deck, blades or no blades) engages, it nearly shakes you off mower. Not too bad while in motion mowing but when you stop and sit still with deck engaged, it shakes. Any ideas. Is there some kind of balance weights on motor somewhere that might have come loose? Motor runs and idles very smooth with no smoke at all and no oil use. No rattles of any kind! Help!!!