Some bike racks (and I assume other things) use an anti-wobble pin system where the pin is actually a specialized bolt and there's a corresponding nut welded onto the passenger side of the tube. You tighten the pin with a wrench so that the nut binds the tube against the receiver, and then slip a safety clip onto the other end of the pin (which isn't threaded) like a normal pin. The system works quite effectively at greatly reducing the wobble.
Some sort of a wedge would probably work as well but removal might be tricky. I'd be tempted to try a piece or two or three of cedar shake or something along those lines if I didn't intend to remove the rack anytime soon.