The San Rafael Swell is a large area stretching about 20 - 25 miles both north and south of I-70 west of Green River, UT. There are miles and miles of trails available to OHV vehicles. I have always been there on a street legal dual sport motorcycle but the trails are available to UTVs/ATVs with the Utah registration sticker. My three visits have involved camping at the Goblin Valley State Park which is about half way between I-70 and Hanksville, UT, just west of UT-24. The area is in the southern part of the San Rafael Swells. The entrance roads to the state park and campground are all paved as are the roads in the campground area. The roads in the state park area of for licensed vehicles but there is a "side" entrance" to the campground which seems to allow for OHVs. Before you get to the actual state park and campground, there are a bunch of places for primitive/dry camping. For those that have toyhaulers or a way to haul adequate water, these sites are great. The advantage of the state park is that it has excellent restrooms/showers and good water. Once you are south of I-70 or north of Hanksville, there are no services - no cell phone reception, no fuel, nothing except the trails and scenery. Hanksville, about 25 miles south of Goblin Valley, has a couple motels, a couple restaurants, a grocery store and a gas station. There is also cell phone service. If you go north about 20 miles to I-70, then east about 10 miles to Green River, there are all kinds of services available. Get a DeLorme Gazeteer for Utah and/or a San Rafael Motorized Route Map (printed by the BLM) and there are enough trails to make your head swim.
Thanks for the info Tim, been looking for some new areas to ride in. Just finished a week in Eagar, Az. and they have 85% of the trails limited to 50". We we're forced to ride on the dirt roads and a few trails we found. I heard that the Piaute Trail is mostly 50" as well.