Hi Jon,

There are some trails on the NW side of the intersection of Mopac and Parmer. They're pretty light, but can be really muddy depending on the weather. It's a fun place close-in. I work close to there, so if you want to try it out, let me know. I'm sure there are some other locals who may want to go as well (cough, cough, Kraig, cough).

Other than that, there are some trails by the San Gabriel river (both N and S forks) in Georgetown. Those are pretty much 4x4 only if you go very far on them.

Hope that helps.