A vehicle is expected to exhibit omni-directional locomotion capability to provide improved rough terrain vehicle functionality. Generally, rough terrain vehicles are not holonomic and cannot travel in the lateral direction, whereas typical omni-directional vehicles have difficulty in traveling on rough terrain. This paper proposes installing a crank leg for a Mecanum-wheeled vehicle ("Mecanum crank") to enhance its rough-terrain locomotion. Compared with other holonomic vehicles designed for rough terrain locomotion, the proposed design exhibited superior capability in the longitudinal wheel direction. To avoid the conflict between the crank leg and Mecanum wheel, this paper proposes the use of a differential gear system. The simple structure of the crank leg enables the implementation of a swing equalizer and roller grousers, which further enhance its rough-terrain locomotion. In longitudinal locomotion experiments, the proposed mechanism could climb a 95 mm-high step, which is 95% of the Mecanum wheel diameter. Furthermore, Mecanum crank could climb miniature scale stairs and execute lateral locomotion on them.