The pilot was performing a high speed taxi prior to an intended local flight. During the high speed taxi, the airplane's tail rose off the ground and the pilot reduced power. During the subsequent rollout, the tailwheel started to shimmy and the pilot applied left and right rudder. The airplane veered left and exited the runway, went down a steep slope and impacted the side of a drainage ditch, where it sustained substantial damage to its wings and fuselage. The pilot reported no mechanical malfunctions.