Extruding/Transforming a Diagonal Face.

How can I easily manipulate a diagonal object. I want to scale it in a certain direction but it locks to axes that make it scale in strange directions instead of parallel to the selection I have. Is there a free tool/addon to make this easier to accomplish?

When I simply select out of plane surface, it scales on its own plane. Axis doesn’t come up. My edit mode is set to Pivot at median point with Global axis. Do you have the same setting?