I posted a question some time ago about vibrations in the frame of my road bike (a 1973 Sekine). Recently, I determined that the rear wheel is sliding back and forth across the axle, with about 0.5cm of play in it. It takes a little bit of pressure to move it, which is why I didn't notice it before (the wheel spins just fine when held off the ground), and I suspect that it is getting worse over time.
My question is: What does this mean? I assume it's not good, but I'm not clear on what has happened exactly. Is this due to wear on the freewheel? Is it just something I need to tighten?