A suspension system for the rear axle of a self-propelled agricultural windrower. The axle is split into two components, each having an air spring to absorb shocks. The two air springs are interconnected by a tube to equalize the internal pressure. A pair of pins can be inserted through the respective axle components and a structural member of the tractor to fix the components relative to the unit structure, thereby incapacitating the suspension and permitting effective steering of the tractor in the absence of a header.