They've got a judgement, and they can seize almost whatever assets they can find. I can promise you they didn't just show up and say "hey, let's grab that bike". Someone has researched it, found you have clear title, and waited for the right moment to get the sherrif to serve you. The lesson to learn here is to be VERY CAREFUL what you co-sign on, because it the other party doesn't pay, you will. Regardless, you need to deal with it, as this won't be the last thing they try to come get. If I were you, if I had a checking account, I'd move it TODAY, because in all likelyhood they will serve it too if the find it. I now it sucks, but you assumed the debt when you co-signed and you are responsible.
I'm suing a father and son right now for $16K that they failed to pay us. I have their personal guaranty. I've been fooling with this and paid over a thousand in attorney's fees to get a judgement, and you can dang well bet I am not going to lost a moment of sleep when they show up at his house and start loading stuff on a truck. I am a cold heartless bas**rd when it comes to trying to steal from me.
I had a guy once who stuck me for about $20K; I had his and his wife's guaranty; they got divorced. When he refused to pay and told me to 'shove it', I sent her a certified letter reminding her of the guaranty and that if HE didn't pay it, I'd be sueing her for all of it too. I had my check in a week; he told me he'd never buy from us again, but yet, here he is 5 years later buying (and paying with cash because I'll never give him a chance again).