Another inexpensive, but very handy option are the NatGeo Topo maps ...
I have every one from just north of Plumas County to south of Rt 4 in Stanislaus County.
802 - 808 IIRC. There are more trails in the Sierras than you can even imagine.
I've been to Prairie City OHV ... it's a park ... with obstacles. I can be on a real trail in another hour of driving so that's what I prefer.
I've also been to Hollister Hills. There are many trails cut through the park, all rated (but may be missing sign posts) as well as several obstacle areas. It can also be full of yahoos. Drunk yahoos. At 10:00am. For me, I can be in the Sierras in half the time it takes to get to HH. However, for $10 you can go and try out your rig to test your capabilities to whatever extreme you feel comfortable with.
Overland bound? .... He suggested to add more lights.
*snort* Don't get me wrong, those guys do have fun ... very controlled, pay to play environment. Excellent group to get your feet wet as they will coddle newbies along to ensure they learn. They do have monthly meetings around the area to discuss offroad topics although I've not yet attended.
Note to Self: seems there are quite a few new member from around my area ... I should put a Meet/Greet together soon.