An employee was removing the outside dual wheel on a farm tractor to get access to the inner dual tire. The employee had the rear wheel of the tractor raised up with a jack and was using a truck-mounted boom to help remove the outside wheel/tire assembly. After the employee unbolted the wheel and maneuvered it off the axel, the wheel tilted and struck the employee. The employee fractured five ribs, punctured a lung, and suffered a laceration behind the right ear.