After finagling with them for awhile, I managed to take my Explorer from not rolling at all, to rolling almost freely with minimal wobble, along with the rest of the Fords I scored yesterday. How did I do it?
It's all in the tires.
You can carefully peel the rubber tires off the wheels of the vehicle, and inspect the tire for bulges and chunks left over from the molding process. With an x-acto knife or even a dremel, you can carefully cut or grind away the excess rubber to bring the tire closer to true. I've found most of the irregularities in them are on the inside of the tire, the part that mates to the rim of the wheel, so you have to turn the tire inside out to work on that part. With some patience and a steady hand, you can make the cars roll MUCH smoother. Unfortunately the molded rubber tires are mostly flawed as a result of the molding process in general, and seem like they would be very difficult to get perfectly true. So I'm researching the possibility of applying this tire-making process to them, which would give them slick (but true) tires:
But so far, the tires appear to be key to making these very nice castings roll as well as they look.