i know of a small place off of HWY 50 E. you take the Cameron Park exit and make a right at the end of the ramp. then take the very first left at the light and go down until you hit the dirt. I'm not sure if it's legal exactly but I've been there lots of times and I've seen plenty of people out there as well.
In fact, heres a video of me spotting my wife on one of the more difficult spots out there;
This is a pretty fun spot if your up this way. I used to live with my dad up in Cameron Park and we would take our Jeeps out here and mess around. Fun place to wheel with friends you can follow a bunch of little trails.
There used to be more fun trails up in the Cameron Park area but they have been gated and closed due to Yahoos coming up and disrespecting the land by leaving trash, driving off the desginated trails and having poor decision making surrounding bonfires. Wherever you go tread lightly.
Can try LaGrange Park, I haven't been there in years but is was OK when last there. Also can try Coral Hollow between Tracy and Livermore, sone big climbs there.