The rear axle in the old wooden oxcart had only one job. The axle was stationary and had no function other than to support the load. When automobiles were designed the design had to include a way to transfer the power of the engine to the rear wheels. Spinning shafts were slipped into the center of the main axle bar at each end and these spinning parts are called the axle shafts. They spin the wheels by transferring engine power. Popular axle shaft manufacturers on 4WD include G2, Crown Automotive, Rubicon Express and Alloy USA. 4WD carries Jeep axle shafts for Dana 25, 27, 30 JK, 30 Reverse and 30 Standard.