On June 29, 2008, about 1250 central daylight time, a de Havilland DHC-6-200, N203SF, owned by Desert Sand Aircraft Leasing Company Inc., piloted by an airline transport rated pilot, sustained substantial damage during descent when the airplane's flight controls bound near Baldwin, Wisconsin. The pilot declared an emergency. The pilot and 14 parachutists departed on the parachute operations flight. Twelve parachutists exited the airplane before the incident. The pilot and two remaining parachutists reported no injuries. The flight was operating under 14 Code of Federal Regulations Part 91 with no flight plan filed. Visual meteorological conditions prevailed. The local flight originated from Baldwin Airport, near Baldwin, Wisconsin, about 1240. The pilot's reported he had initiated an idle power, descending tight right turn to a heading of 090 degrees. The airspeed before the turn was approximately 70 KIAS [knots indicated airspeed] and at rollout less than 100 KIAS, which was below the maneuvering speed of 126 KIAS. During the roll out, the ailerons where effortlessly brought to about 7/8 of full left travel at which point it felt like some part went "over top dead center." The pilot made an immediate full force attempt to bring the ailerons back to neutral but failed. The pilot said that a lot of play was apparent in the aileron controls. With full right force on the yoke, the plane appeared to not roll left. The passenger in the copilot seat was told that a problem existed and was asked to help push the wheel down on the right. The combined effort allowed for a non-rolling flight. Throughout these attempts to control the roll, the nose down attitude was sustained. Elevator control was not affected, but bringing the nose up raised the concern of inducing a left hand roll. Before reaching approximately 4,000 feet mean sea level (MSL), 3,000 feet above ground level (AGL) the pilot raised the nose of the airplane. Subsequently, the aircraft started to roll left. A maximum force input did not level the wings. As the aircraft reached 90 degrees of left bank, another more concerted attempt at controlling the aircraft was initiated. With the airspeed slowed, full power on the left engine, full right rudder, and full force right aileron input, the pilot was able to establish nearly wings level, straight flight. The pilot made "mayday" calls, which were relayed by another airplane. The airplane diverted to Saint Paul, Minnesota. The passenger in the copilot’s seat, unable to no longer provide physical input to help maintain control, was replaced by one of the parachutists sitting in the back of the airplane. Once the parachutist was in the seat, the pilot asked him to "give it all you've got." His input together with the pilots’ brought the ailerons back to neutral. The pilot stated that there were no observed anomalies with the airplane prior to the flight. An examination of the airplane on-scene, to include the aileron and flap systems, showed damage consistent with an in-flight overload.