On January 5, 1999, about 1425 Alaska standard time, a Piper PA-31-350 airplane, N6GR, sustained substantial damage after being struck by a vehicle at the Anchorage International Airport, Anchorage, Alaska. The airplane was taxiing to the operator's facility after landing. The airplane was being operated as a visual flight rules (VFR) on-demand passenger flight under Title 14 CFR Part 135 when the accident occurred. The airplane was registered to, and operated by, Jim Air Inc., Anchorage. The certificated airline transport pilot, and the five passengers, were not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. The flight originated at the Chenega Bay Airport, Chenega, Alaska, about 1340. The Director of Operations for the operator reported the airplane was taxiing on the Lake Shore taxiway, located along the west side of Lake Hood. A snowplow equipped pickup truck was removing snow from Lake Lot 75, a private airplane tie-down spot, located alongside the taxiway. The truck, operated by the owner of Lake Lot 75, was equipped with a metal lumber rack in the bed of the truck. While positioning the truck, the owner backed across the taxiway, into the path of the accident airplane. A portion of the lumber rack struck the outboard end of the left wing of the airplane. The airplane received damage to the outboard 3 feet of the leading edge, including several wing ribs. The Anchorage International airport permits limited vehicular traffic on Lake Shore taxiway, primarily service vehicles and aircraft owners. It is not open for use by the general public. There are two signs located about 50 feet from the point of collision. One sign that faces south reads, "Caution Aircraft Operation Area - Yield to all Aircraft." A second sign that faces north reads, "Aircraft Only - No Vehicles or Pedestrians on Taxiway."