Most junkyards can tell you what rear axle assemblies will interchange. Probably easier, maybe cheaper to swap out the whole deal than to rebuild the ring/pinion/bearings. Dunno for sure.
If thorns and sharp rocks are not a concern, go radial tires, and play around with air pressure. You can run soft, but not too soft. 30 to 35, front; maybe as low as 25, rear, unloaded. I don't recommend this for 80mph in August, of course. (Main reason for those Explorer tire failures: Underinflation, high speeds, hot days. I like the free tires, of course.)
Take out the bench seat and get some junkyard seats from some small sedan--try various ones until you find something that suits. Most of these will give you more leg room, and you can vary the angles. You'll have to fabricate new brackets, but that's no big deal.
Heating the back? Hmmm. Maybe a temporary fixture to hold a 12-volt squirrel-cage heater fan, and some ducting. Set the fan where its intake is right at the discharge of your regular heater...
So there's a start.
, Art