The pilot was using soft field procedures to takeoff from a 1600 ft dirt/gravel strip. The strip was wet and soft from melting snow, it sloped downhill with a 3% slope and 15 to 20 ft trees were located at the departure end. Also, the pilot reported there were 3 'good size heaves' in the runway. Shortly after becoming airborne, the stall warning horn sounded and the aircraft settled back on the runway. Subsequently, the pilot initiated abort procedures, but was unable to stop on the remaining runway. He maneuvered to avoid a van, then the aircraft nosed over on rough uneven terrain. The no-wind takeoff roll for hard surface was estimated to be 1100 ft with no obstructions and 1970 to clear a 50 ft obstruction. The pilot suggested there was an engine problem, but the passengers noticed no loss of power. Also, the passengers said that during the takeoff roll, the stall warning horn was on and the tail was dragging the ground. Scrape marks were found on the underside of the tail.