On December 2, 2011, about 1228 eastern daylight time, a Cessna 560XL, N576CS, landed at Orlando International Airport (MCO), Orlando, Florida, following an in-flight pitch trim malfunction that occurred near Ocoee, Florida. The airline transport-rated pilot and co-pilot were not injured. The airplane was not damaged. The airplane was registered to CitationShares Sales, Inc. and operated by CitationShares Management LLC under the provisions of 14 Code of Federal Regulations Part 135 as an on-demand air taxi flight. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed. The flight originated at Naples Municipal Airport, Naples, Florida, at 1150 and was destined for Washington Dulles International Airport, Dulles, Virginia. The flight crew reported the following. While approaching their level off altitude of flight level 410, a "Pitch Trim Miscompare" message illuminated in the cockpit. After accomplishing the checklist items and disconnecting the autopilot, considerable forward yoke pressure was required to maintain level flight. The crewmembers found the manual pitch trim control wheel to be "frozen" in the forward position and were unable to move it. The crew declared an emergency and diverted to MCO. During the descent for landing, upon reaching 8,000 feet mean sea level, the trim wheel released and the system returned to normal. The crew cancelled the emergency and landed at MCO uneventfully. After the aircraft was secured, maintenance personnel inspected the flight control system. The grease in the left and right elevator trim actuators was found to be contaminated with water. The trim actuators were replaced and the airplane was returned to service. According to the operator, the elevator trim actuators were last inspected and lubricated 562 hours prior to the incident. The inspection requirement for the actuators was at 2,400 hours initially and at 1,200-hour intervals thereafter.