Can I take legal action against my husband if he ran off to Mexico and married a woman he got pregnant there?

There are some things which might be done, but they all carry costs. You can continue the American divorce, and if he continues not to appear, you might be able to receive a good deal more than half the marital estate. You could notify law enforcement that he has committed bigamy although the offense is probably under Mexican rather than American law. If Mexico has laws similar to ours relating to recognition of foreign judgments, you might try to begin an action to have an American court determine that his second marriage is illegal in the US, and you might want to hire a Mexican lawyer to do the same thing in Mexico. A skilled lawyer with experience in transnational divorce could be a great help. Good Luck