What capability does the rear axle provide in terms of motion?

The rear axle on a crane is designed to enhance its maneuverability by allowing it to pivot. This capability can be crucial in tight spaces or when navigating around obstacles. The fact that it pivots four inches in either direction means that it is able to provide additional flexibility, helping the crane to make sharper turns or adjust its position without needing to move its entire frame. This adjustment is particularly useful when the crane is performing tasks that require precision in positioning, as it enables better control over the load being lifted or moved.

In contrast, the other options refer to functions that are not typically associated with a rear axle's purpose or capabilities. Steering angle adjustment implies a different mechanism designed specifically for altering the direction of the vehicle, while vertical movement pertains to elevating or lowering the crane, which is usually managed by the main lifting system. Lastly, locking the axle in a fixed position would eliminate the benefits provided by the pivoting motion, making it less versatile in operations that require adaptability.