Yo, Dr. Ingineer! If you're willing to drop a third axle on it, how about instead making an adjustable mount for the existing two-axle, a la pin-adjustable semi-trailers? instead of a fixed location for the leaf springs (I assume leaf-spring; is it torsion bars?) you make a slidable, adjustable locator system like they use in them-thar "big rigs". That way you can slide it backards and for'ards to meet your load....

GA

P.S., Luvin' that solar panel suggestion. We dropped one on ours mid-winter and batteries are "A-OK"...thanks for the suggestion!