I lived in south florida a few years ago, and I would always go to Big Cypress Natual Preserve. I don't know if they still let you wheel there, but it is FUN. That is just west of Ft. Lauderdale off of 595 or SR 84 (if memory serves me right.)
Also if you go south of Homestead/Florida City in Dade County (like if you're going to the Keys) go on the alternate route (Card Sound Road)and just after the road begins, there are some trails to your left -- mostly ATV's but some Jeeps and swamp buggies.
There is also Loop Road just before Shark Valley on the Tamiami Trail (US 41) in extreme western Dade County. Loop Road is good for 2wd vehicles too during the dry season, and it's about a 20 mile trail and some nice scenery of the everglades. (my son used to love it because there's gators on the trail!)
For south Florida, that's about it, because all of our old spots are now subdivisions!
Check with Big Cypress before you treck out there because they might require a permit be purchased in advance.
Hope this helps!