I-551's without an expiration date are still valid for all purposes. However, you should be prepared to be hasseld by a CBP officer upon your return about replacing this very old card. USCIS has already indicated that it will soon require all I-551's without an expiration date to be replaced so it is in your best interest to do this sooner rather than later. Since you have obviously been a resident for more than 5 years you may also want to consider applying for naturalization.
There is still no legal requirement that persons with the older style green cards that have no expiration dates printed on the cards replace the cards.
The federal government is still strongly urging holders of older style green cards to replace the cards as the newer style cards have security features that the government likes.
Aliens with any history of criminal law violations should be very careful in leaving the US as the aliens may be placed in deportable proceedings upon returning to the US if the violations are basis for deportation. The inspectors at the ports of entry now have access to a lot of information that used to be available only to local law enforcement agencies.