It depends on the amount of tread depth you have on the remaining tires compared to the new tire. If the difference is 5/32nds or more I would consider replacing all four. If it’s just 2/32 or so, I would recommend mounting in the inner wheel position as that will wear it down quicker to match the outside tire (Road crown).

As to the axle, you have a floating axle so chances of the axle being bent is slim. Now, depending on the impact, while I doubt it happened, the housing “could” have been tweaked. I would definitely have an alignment done, a 4 wheel thrust angle. If something has moved they should be able to tell once the initial reading has been taken. Make sure you ask/require both before and after printouts.

There could more damage than just the tires and wheels. My buddy's Wrangler got t-boned directly to the rear passenger tires. End up replacing both rear shocks, both side tires (5/32 tread depths after 1.5years), and a couple of alignments to get back in shape. He later decided to make it a weekend trail toy and got another car as DD. Now the Wrangler got Bilstein shocks, nitto ridge grappler tires, and skid plates.

If it’s the other driver’s fault have their insurance replace all four tires.