I'm got my vehicle setup using this Tutorial http://www.gotow.net/andrew/wordpress/?page_id=78 i found on this site.right now the vehicle is moving but the wheels seems to be jumping out of the vehicle why is that.
I have used the same project before and it works fine, so how about you take that project as a base and replace the model with your own model. If you still have problems maybe there's something wrong with your model.
Good luck!
Make sure the origins of your models are at the origin, and not offset at all. I had this problem too. So ffr, when making wheel models, build it around the origin, or make it and centre it then export.
Another problem I've come across with that project is that the wheels do exactly that when your car has a scale which isn't 1,1,1. May not be that, but worth checking