You will need a Passport or the new Passport Card to cross back into the USA from Mexico. If you don't have one already and don't really see any international air travel in your future or wanna save a few bucks, get a "Passport Card" for $55. Here's what the Dept. of State says about 'em and here's the link. Processing time is about 4-6 weeks.
The U.S. Passport Card can be used to enter the United States from Canada, Mexico, the Caribbean, and Bermuda at land border crossings or sea ports-of-entry and is more convenient and less expensive than a passport book. The passport card cannot be used for international travel by air.