You can try this site, it mainly lists ATV and bike parks, but there are some for 4x4s, ATV Trails, Motorcycle Trails, Motocross Tracks, California
. You might also try the Florida section of the forum and/or a local club. You can also check with the national and state forest offices, sometimes they have trails that are open. There is also a Florida Maps and Gazetteer book that you can get. I've got one for Kansas and Missouri, they usually list the fire service roads, as well as some of places that only locals know about.