I would go for a two-wheel-drive delta trike, with jackshaft drive to each rear wheel rather than an axle with a differential. If one drive wheel loses traction, you still have drive at the other. With no axle between the wheels, the cargo box could be lower to the ground to improve stability. Fit 24"-26" wheels with fat tyres for muddy terrain and it should be quite capable. A front hub motor could be useful for hauling heavier loads and would also give you three-wheel-drive where needed.