Upton Engineering in New South Wales, Australia built the world's largest two-wheel drive tractors in in the 1970s. Early Upton tractors in the 1960s used military-surplus tank components. Later Upton tractors utilized truck construction equipment components, leading up to the Upton HT-14/350; a 51,000 pound two-wheel drive tractor fitted with earthmover tires. Upton stopped building tractors by 1980, although the company continues to build irrigation equipment.
Upton Engineering - official site
Chamption for 2WD - 1979 Power Farming article at Puddings World