Need to ask a few questions before trying to solve this one. 1. Does the engine vibrate? 2. Does the deck vibrate? 3. Does the entire unit vibrate? 4. Does it vibrate only when engaging the deck? 5. Did this start all of a sudden or has it slowly gotten worse? 6. How bad is the vibration? 7. When was the last time you replaced the belts, sharpened the blades, checked the spindles and leveled the deck? There are many different things that can cause a unit to vibrate and we need to narrow things down.