Planning to drive from HRL to about 2 hours by car into Mexico. Do I need a passport or will a valid driver's license and social security card suffice?

To do what? To go deeper into Mexico you'll need proof of US citizenship & payment of the tourist fee. This won't apply if you stay within 20miles or so of the border. US social security cards are irrelevant in Mexico.

You will need a passport to drive to Mexico/Canada starting the middle of next year

Incorrect. You do NOT need a passport to "drive into Mexico" - if you are a US citizen, a more basic form of proof of citizenship will suffice. If you stay on the border, you most likely don't even need anything.

Trying to get back to the US is another story, but to go INTO Mexico you do NOT and for the forseeable future WILL NOT need a passport if you are a US citizen.

Wrong for land and sea travel as of Summer 2008:

SUMMER 2008
At a later date, to be determined, the departments will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have either a U.S. passport; a U.S. passport card; a trusted traveler card such as NEXUS, FAST, or SENTRI; a valid Merchant Mariner Document (MMD) when traveling in conjunction with official maritime business; or a valid U.S. Military identification card when traveling on official orders.

The implementation date will be determined based on a number of factors, including the progress of actions undertaken by the Department of Homeland Security to implement the WHTI requirements and the availability of WHTI compliant documents on both sides of the border. DHS and DOS expect the date of full WHTI implementation to be in the summer of 2008. The precise implementation date will be formally announced with at least 60 days notice.

As of now you can show your driver's license, but that will change starting in 2008.

Incorrect. I stand by what I wrote. The question is about entering Mexico. Mexican law DOES NOT require US citizens to have a passport.

Has nothing to do with entering Mexico

This being the Mexico forum, the OP's question is about entering Mexico, not entering the US, so I repeat that you DO NOT need a passport if you're a US citizen.

True, but you will to get back into the US next year. If they live here and want to come back then they'll need one.

Passport requirements aside, don't forget the requirements to bond your vehicle with Banjercito once you get into Mexico. 2 hrs. into Mexico from there sounds like Monterrey...wherever it is, it's beyond the exempt zone for the bond, and even though there's a million cars in Nuevo Leon with Texas plates, if the police are feeling picky enough, you'll be in big trouble without the bond.