It could be that your wheels have become warped through hard impacts. if you hit stuff hard enough, wheels will either break or warp. This seems most likely, but I guess it could be possible that the axles could be bent as well.
There are many other things that it could be, but I would go through your wheels and suspension and driveline parts that could be causing this to happen. This something that we can really only guess about without having your car in our hands.