There's the park at Juniper Springs which has a big area with grills, pavillions and such. Joe had the June 2003 VX meet there and much mud was to be had, as -X- and Heraclid can vouch for. There's even an "official" DVD from the meet! Mud not included. LOL!
Here's the photos: 2003 Ocala VX Meet
Blue Springs (Volusia County) might be a good choice too, but there's not much in the way of off-roading. The spring is nice though and open to divers if any in the group were interested. You can make about 110 feet down into the main spring.
Just a couple ideas!