I was mowing last week when the rear wheel - on the drive side (right)
fell off. No warning, the rim stripped off and broke. I ordered a
replacement wheel/rim combo.

How in the name of Sam Hill do I get the old "rim" off? I put some
liquid wrench on the bolt end to help free it up, but there's nothing
for me to grasp to free the bolt that holds the wheel on. I tried a
pimp wrench, vice grips, etc. I try to turn it and it slips right off.

Any tips?

FYI, what's left around the axle is the plastic base of the hub.
Unlike the front rims, the back rims seem to be bolted directly to the
axle. When I turn it, the axle turns. I haven't removed the "hogshead"
to see if there is something that I could clamp onto.