[freebeard]

Yes, I'm planning to drive the axle, and I'm picturing the most reasonable axle to use for my configuration is a straight axle. As I've read through the replies, I think finding an appropriately sized manufactured trailer is the best path. Then, swap in a solid axle that matches, and work on a modified trailer instead of a scratch-built trailer.

Your search term is American Underslung.

I expect the easiest way to control the load is to drive the system with an electric motor, but the least expensive way will be with a small gas engine. I'll be starting the design effort soon, and will see if any nice electric power options come up on the used market.

I gave it my best shot at Permalink #12. Any hybrid SUV with an electric rear axle will have a complete drivetrain and suspension as an OEM pre-engineered unit.