Hate to be the bearer of bad news, but the odds are that the severe vibration that you are experienceing is not due to an out of ballance blade. When a mower hits an object like a root, or rock or other obstruction, it often bends the crankshaft that the blade is attached to. This happened to my parents Honda mower when my son ran it into a rock and the repair guy told us that it is a common occurance. In their case it required a fairly expensive repair, the replacement of the crank shaft AND a new blade. You can try a new blade, and maybe you will get lucky, but if he hit rocks numerous times, I expect that the vibration will still be there with the new blade. They don't make the motors on mowers with heavy enough parts to be mowing rocks.