Upvote:-3
As of March 1, 2010, U.S. citizens and other non-Mexican citizens need passports to enter Mexico. You do not need a passport to enter Mexico if you're a U.S. citizen who is visiting a port town for a day and have come by sea, such as on a cruise.
Upvote:4
US Citizens do not need visas for tourism. They are granted a (usually) 180-day visitor permit upon arrival in Mexico. Just bring your passport.
If the purpose of your trip is business, immigration, etc, then you need to apply for a visa at your local Mexican consulate prior to travel to Mexico.
You can read more here.
When you arrive in Mexico (in Guadalajara), your wife could, in theory, go through the "Mexican citizens" line for immigration, but instead, I would suggest your family go together through the "Foreigners" line, and explain to the agent that she has dual citizenship.
When re-entering the US, your wife must present her US passport, as all US citizens are required to do.