Basically, I ended up gerry-rigging it. I get my deduction as if I didn't sell it, then I go back and make sure those boxes are checked and X out of it/switch sections before I hit "continue" where it wipes it out. As far as I can tell, I still have my gains reported on the car (it was gifted to me, so I don't have an initial buying price issue), and I still have it reported that it's sold, but I still have the mileage deduction. I don't think my forms will report any false information.
