There are no legal ones, except for the kid's trail next door to his house on W Pt Washington Rd.
I've seen one on Hwy 20, well east of Freeport, closer to PC. I also read that one of the dirt pits in NoWal, not sure where, was going to stop being used for a pit, and would become a motorcross place.
Though it is not a trail, I believe laws in FL, changed a few months ago, allowing some ATVs to legally be ridden on dirt roads, but I recall some opposition to the change in the law, and there were restrictions on the riders, regarding age, speed, etc.