If all you got was a letter saying the costs MAY exceed your policy limits, then you don't need to worry yet. If you are served with suit papers, that's when you can talk to a lawyer, but first you should talk to the lawyer your insurance company will hire for you (since you won't have to pay extra).
If you have assets, you are not judgement proof and there is no way to (legally) make yourself judgement proof. It was irresponsible for another poster to suggest otherwise.