So I found a company that has been making a 'ditch plow' for over 50 years . . . it looks like it would work great with non-concrete type of ground.
It throws the dirt equally to both sides of the swale, which isn't what we want. But I am sure with a little ingenuity, we could modify it to throw it all to the downhill side. It says it can dig a swale (ditch) 7 1/2' wide and 36" deep. Or smaller than that based on how deep you push it in the ground. It is meant to be pulled behind a dozer of some kind. Probably the heaviest dozer you could get your hands on for more effective swale creation.
I'd like some 2nd and 3rd opinions on this piece of machinery. Anyone have any experience with anything like this?