Are you talking about the National *Forest* area mostly? I think the National Park is fairly restricted. When my duaghters and I went, I had forest service maps of the Sierra NF and the Sequoia NF and just explored. The maps indicate the road conditions pretty well:
http://www.aliensonearth.com/catalog/detail/0123/0123K806218.html

The Kern Plateau area is good:
http://www.kernvalley.com/news/offroad.htm

Kern Valley OHV guide with NF trail numbers:
http://www.kernvalley.com/news/platohv.htm

We found quite a few good challenging roads in the area south of the "Trail of 100 Giants" area (generally south and around Johnsondale). Bring gasoline if possible; there are few gas stations in SNF, but those that exist are pretty expensive if you keep filling up there. I think Ponderosa was about it for the north loop. The food there is good though!

Parsa