Reaching a height of 75 feet after takeoff severe vibration was experienced. The aircraft then entered a nose down attitude. The flight crew could barely prevent the Convair from diving into the ground; they managed to carry out a belly landing in barren field. PROBABLE CAUSE: “Loss of control due to a failure of the right elevator trim tab push-pull rod caused by a reversed installation of the right elevator trim tab idler as a result of the carriers reliance on Manufacturers Illustrated Parts Catalog as a maintenance reference.”