The problem is a worn out hitch. I deal with a fleet of trailercycles, for rent, and every year I replace the hitches due to wear. Similar to how a chain wear out by the plates, pins, and rollers rubbing together, your hitch wears from the parts of the universal joint rubbing together. Check with an LBS to see if a replacment hitch is available. I wouldn't worry about your seatpost, it is more then strong enough to handle the stress.