It seems that the availability of trails widely varies depending on where you live. Here are a few links that show trails according to state or regions. But, you have to do your research and find out if in fact off-road vehicles are legally allowed on the trail:
Off-Highway Drives | Off-Road Trails
4-Wheeling Trails and Maps for 4WD Trucks and SUVs - Where To Ride 4x4s Off Road
In addition, jeepjamboreeusa.com offers offroad trips all around the country. However, a single trip with them can cost about $500. It's worth checking out the trips on their site though.
You can also join your local Jeep or offroad club. These clubs usually require membership dues and take part in offroad trips.
Hope that helps.