How well would your waterjet cut plastic styrene?
A 4x8 sheet of .100 might be cheaper than steel.
If this trk is 80-90lbs when finished, then add another 50-60lbs of dirt to haul... that lil banebot motor might just aint gonna do it?!
A tube axle with a banebot motor in each side to direct drive each wheel... hook both motors to a Sabertooth dual esc, with the mix on & the steering lead Y'd with the steering servo lead, that might work better than a differential itself & give you tonnes of grunt!