Ok, another update! I found a set of rims and AT style tires from a Chevy 2500. The hub ID for the chevy rim isn't quite right, but they bolted up ok with a spacer on the backside. The tires on these are 245/70R17, which are around 30.5" overall height. They are load index C tires, so only good for 2200 lbs each at highway speeds, but with in the realm of the likely loads I'll be experiencing with my machine. They do make the machine a bit taller from the R4s I had on it before, but the traction is noticeably improved with the new tires inflated to 20 PSI or so. They also don't damage the lawn, whereas the R4s caused issues during turns.
I'll post back with my experience over time.