I overstayd my J1 visa 8 years ago. Any chance to become legal again ?

It depends - speak with an immigration lawyer.

Normally, the problem people who are now out of status have is this: they can't get a green card in the U.S. without leaving because of the lack of current valid immigration status (unless they entered legally and marry a citizen, or one or two other very rare circumstances), and in most cases if they leave the U.S. to process through a U.S. consular post in their home country, the act of leaving makes them subject to a three- or ten-year bar to coming back because they have accrued 180 days or one year, respectively, of "unlawful presence" in the U.S. before leaving.

For a person here without valid status to be able to get a green card, one of the exceptions needs to apply. Either they can get a green card here because they entered legally and marry a citizen, they are eligible for something called "245i" and can get a green card here anyway, there is some reason why they can leave and process abroad and NOT be subject to the three-or ten-year reentry bar, or they can leave and be subject to a bar but have a good case for a waiver of the bar.

I think there might be a chance that you fall into the "some reason why they can leave and process abroad and NOT be subject to the three-or ten-year reentry bar" exception. Usually, people start building up "unlawful presence" as soon as their I-94 card expiration date arrives, and they stay here.

However, people admitted on I-94 cards marked "Duration of Status" ("D/S") don't really have a firm I-94 expiration date. Unlawful presence time doesn't build up for these people unless there is a formal determination by a government official - immigration officer or judge, usually - that they are out of status.

Most people on J-1 visas - like you - as well as F-1s get I-94 cards marked "D/S." If there has been no formal determination that you are out of status, you might not be subject to the bar to reentry and so could process abroad for a green card without a three- or ten-year bar.

Speak to a lawyer to find out for sure, and see what options you might have for permanent residence.

Good luck!