The pilot reported that on takeoff, the rudder jammed in the full right position and he was unable to free it. The pilot attempted to pull up to clear trees adjacent to and right of the airstrip, but was unable to clear them. The airplane crashed into the trees. No problems were found with the rudder control in post-accident examination. The pilot stated he believed a canvas briefcase behind the pilot's seat may have moved (or been moved) to one side or the other, possibly creating an obstruction to the rear rudder pedals.