You have several areas close by, go to your nearest Offroad shop and pick up a trail guide, you have to check out Superstition Mountain, Ocotillo Wells, Truckhaven and Borrego Springs in the fall. These are excellent parks within a few hours from San Diego.
There is also the Sawdust Jeep outing or something like that (don't quote me) in I believe in October at the Salton Sea, hundreds of jeep club show up for that deal. I will definitely check it out myself this year, as I just switched from a Prerunner to a Jeep and could not be happier.
Heres a link to Cali Jeep Clubs clubshttp://www.jeepsonly.com/Clubs_and_Associations/By_State/California/