As far as wheeling goes in San Diego, I know of a few spots, I lived there for about 5 years.
If you ever get up to Julian, there are quite a few trails around that area. Its kinda a backwoods place to be only a hour or so away from the city. Nice people, great dining, and even better trails. Best thing to do is go East on Hwy 8 for about 45 minutes (past El Cajon) . That will put you in between the mountains in the Cleveland National Forest, and that my friend is the mecca of rock trails. I can't count how many weekends I have spent in that area. If you dare to travel futher east, you'll be in Imperial valley. Mostly just a huge Sand box, but if you like to play on the sand dunes, then Glamis will give you a huge

. Just ask around and join some clubs. Southern California is where its at when it comes to four wheeling. oh and don't forget Baja. I've been down to Ensenada, San Felipe and some of the other small towns in Mexico. You can pretty much pull off the side of the road (what little pavement there is) and venture out. Just make sure you have a understanding on Spanish and have plenty of Spare tires.
All in all, just be adventrous! San Diego county has so much to offer. I wish I had my crusier when I was there. I did most of my wheeling in my two tacomas, which are about as popular as H2's

out there. Have fun man and if you need a virtual tour guide hit me up, hell I might even fly out there to join you, I miss it much!
Stuck in South Carolina