Maintance help: my back wheel seems loose on the axle. what can i do?... (details)?

The first thing to do would be to check your quick release on the wheel. If it's loose, tighten it. If that doesn't help, your wheel bearings are probably loose (or worn out) and need to be tightened. If you have the specialized tools for hub maintenance do it yourself or you can take the wheel to a shop and have them look at/fix it.