There's a really good reason why most people change the axle...
I would check with military truck and surplus vendors like Boyce equipment for wheel/tire stuff. Stockton Wheel in California also does some military lug wheel conversions. They might be able to make a wheel that would accept a more common tire if you're stuck on keeping the axle.
I helped a friend "civilize" an M105 once. We actually just got rid of the chassis and welded spring hangers and an A frame tongue straight to the bed. It was still stupid strong and heavy as the steel is really stout on those. Basically it became a uni-body trailer which lowered it down quite a bit and fit behind the F250 a lot better. He used an electric brake 5.2K axle in spring over configuration with standard off the shelf trailer springs. It was intended as a firewood hauler and boy did it deliver!
Back then the M105s were still going for $50-$100 at auction so it was a little more tempting to mess with them.