Speaking of big motors and big torques its interesting to consider CNC machine tool conversion technology ... Admittedly continuous use, industrial level indestructibility and temperature levels means expense, but if you want 400+ oz=in of torque for a wheel perhaps, you could do worse than CNC controller hardware perhaps driven by something other than CNC software. At some point you have a 5 volt TTL interface thats as simple as direction and step. You'll be spending maybe $200 per axle but you look at the price of 400+ oz-in servos or gear motors plus drive circuitry and suddenly its not that bad. 1000+ oz-in is not out of line for the larger machine tool conversions. Standard NEMA motor mounts etc. You take 200 steps/rev stepper and add 10-step microstepping driver like a geckodrive and theres 2000 steps/rev at 400+ oz/in with a simple TTL interface for maybe $200 (plus power, and steppers are hungry beasts)