ok, so you didnt live at that address since 2003, and they served you there in 2004. Can you prove this? If you can, then you need to gather documentation of that proof and send a copy of it to the court, and request that the judgment be vacated on the grounds of improper service. Did you have anything changed when you moved back then, like your driver's license, mailing address at the USPS, anything like that? Utilities put in your name at new address? How about a lease agreement or purchase contract for the new place you moved to? Any of these things will substantiate your claim that they had the wrong address. They will also do something else--they will show either intent or negligence on the part of the CA that sued you. you see, if you can prove that you were "in the system" as having moved to the new address back then, then they dont have an excuse. Any attorney has resources to find someone's address if they moved, as long as they have a legitimate job(your SSN would show that you have moved) or any of the things I listed above...and an attorney could get this information pretty easily and quickly. The fact is, they didnt try to make sure at all, they just sent it to an address they had on file. That, in my opinion, makes them liable and the judgment should be vacated.
Also, I dont know how long its been since your grandfather passed, but you may consider talking with the person that was the executor over his estate--they would have had to deal with the financial records, and there's a possibility that you could still get your hands on proof of payment, if he paid them. An old checkbook, or bank statements...something may still be found. It's worth a shot...