If you take property from people who owe it to you, that's stealing?
If you don't go through the right avenues, absolutely. I don't get to go to your house and take your stuff at gunpoint simply because I believe you owe me money, that is theft plain and simple, direct robbery. Especially so for something you owed from back in 1998 (in CO it would actually be illegal to try to collect on a debt over 7 years old). (in 1998 Nevada took over the billing, etc. for the land in question.)
He owes money, not a life.
Put a lien on his property if the laws allow for you to attempt to collect money owed from that far back.