Yeah, it's a dolly. Larger versions are called "Joe Dogs" in certain circles. Keep in mind the heaviest tongue weight of the "stuff" you need to move around. If you build it to carry the heaviest thing you have, the lighter stuff will be easy to handle too. The wider spacing you have between the wheels, the greater will be the stability and resistance to sideways tip over. You need to make the tongue at least 4 feet long or backing up the rig will be tedious and uncertain (it will take some practice to get the hang of backing with this dolly rig). Mount the ball on the centerline between the wheels and maybe an inch or 2 in FRONT OF the wheel axle centerline to add just a bit of down pressure on the rear of the tractor to enhance traction.

Joe Dog. New to me. I'll do a search and see what happens. Thanks!

I don't know how skilled you are in metal fabricating and designing, but click the link below to view just one example that you could base your dolly on. The dolly shown is intended for moving boats on trailer, light camper trailers etc. on paved, level surfaces. It is managed by one or two persons as needed.
The basic design can be easily adapted to allow the moving to be handled by a lawn or garden tractor.
I'm not saying buy one of these and modify it, though you probably could easy enough if you have the skills and equipment.

Here is a link that might be useful: Inspirational offer