DragRigidBody Lock Axis on movement

I am using DragRigidBody to move my game object across a board which works fine. I have locked out the Y axis, so this allows movement in the x and z axis, however what I want to constrain all movement on the x axis to the x only (not allowed to drift on Y), likewise if I decided to move on the y axis, no drifting on X.

Being a novice I have looked everywhere for an answer but either cannot find it or cannot recognise it. Any help greatly appreciated.

To be honest with you the only way I figured out how to do this was to set triggers around the corners of where you want the transition of movement!