This paper proposes a new whole-body motion generator for a quadruped-on-wheel robot that can cross various rough terrains, especially with a much-altitude difference. The motion generator combines the kinematics model, the wheel-center motion model, the robot centroidal momentum and dynamics models, and a proposed altitude-control model. Then the whole-body motion references can be achieved by giving the robot centroidal-motion reference, including the centroidal translational motion in the forward and lateral directions in the inertial frame, and the centroidal-height motion w.r.t. the base frame. Especially, the relative motion depends only on the robot-self property, and is independent of the terrain geometry. The robot is simulated to be driven on various tested rough terrains using our new motion generator and our compliant torque controller.