As above, a sawzall, grinding wheel or possibly a hack saw (although that probably wouldn't cut through a hardened shaft. If you don't have those tools, almost any "service station" or body shop would help you for a small charge.
Something we all can "take away" from your problem is the effects of ice, salt and corrosion on any lock that's "normally under our truck"... That includes spare tire locks, receiver pin locks, tool box locks, and any other lock that's exposed to the elements.
As we're getting ready for camping season, it would benefit all of us to spend a few minutes checking all the locks and "gear/cable/worm" actuated devices on our trailer and tow vehicle. Nothing is more frustrating than having a flat tire and not being able to get the spare tire mount unlocked so you can change the flat and get back on the road... Well, maybe "nothing"....