Anyway, I decided to see how much new casters would cost and discovered they were quite reasonable, with a two-pack of the appropriate casters costing something like $4 at Home Depot. I wasn't sure they would be the right size, but I decided to give them a shot. Shockingly, they are almost exactly the same height as the existing casters. They're a tiny bit shorter, so there's a slight rock to the seat, but it's totally usable. I suspect I will eventually have to replace all the wheels with these, so they will all be even someday.