The White Rim Trail in Canyonlands National Park is more popular as a bike trail than a 4WD route, but almost everybody who travels the length of the trail (90 miles or so) has to have some kind of vehicular backup. The trail is not tough by Moab standards, but many sections are rough and slow. People on mountain bikes make better time than those driving.
In order to do The White Rim Trail, you have to obtain a permit in advance. This can often be the most difficult part of "doing" the trail. The Park Service limits the number of vehicles per day on the trail (because of limited camping sites, and to minimize the impact of visitors). Get your permit as early as possible, because peak periods fill up fast. There are three or four camp areas along the trail, and most people camp two nights. There is some pretty good wheeling in spots, but the incredible views of the La Sal mountains and the red rock surrounding the Colorado are the main attraction. I spent the better part of one day on my bike, and had a great time. A stock 4x4 should have no problem if driven with care. A few steep uphill and downhill sections may cause problems for those with low clearance.
There were four of us and three bikes, plus all the camping gear, water, gas, and food for the three day trip. All this stuffed into and onto the "Hedgehog". As usual, the thing was dragging tail! We got a late start the first day because of some confusion about which day we were supposed to begin the trail, so we ended up really blazing down the narrow switchbacks that descend to the river. There are some amazingly steep and exposed sections of road, which are even more difficult to navigate in the dark. The first campsite wa fairly close, but we still ended up making camp in the dark. I seem to remember some of the gas cans on the roof were leaking onto nearby sleeping bags! The next day was a full day worth of Jeeping and biking to the second campsite. It was nothing more than a flat dirt parking lot, but the views during sunset were amazing. After several margaritas (who says you have to ROUGH it!?!?) I made the poor decision of trying to ride a wheelie on my mountain bike. I went over the back, landing hard on my tailbone, or coxxyx, or whatever the $%#@*&!! you call it.
The final day is an easy one, and if you do the trail in the "normal" direction, you end up at the Canyonlands Dead Horse Point visitor's center. We opted to take the Sheafer trail back to Moab (rather than the highway back toward Arches). The Sheafer trail is where 90 percent of the Jeep commercials must be filmed. Great vistas of the Colorado river, red cliffs, and the La Sal mountains off to the east. We had a great night camping along the trail. The picture below is of my friend Dave and I getting clean (finally) before heading back to Moab.
The trail isn't rated, but parts of it probably merit a 3+. After a big thunderstorm, it could be a lot tougher, and wheeling at night definitely adds to the thrill factor!