Yup, I've done the shuttle myself, not fun and wastes too much time. If you do the regular route any car can make it. It is a dirt road, but it's in pretty decent condition. You don't need 4wd, unless it rains of course.
I've dealt with River Runners for over 5 years now and know the owners personally. They are great people, take care of your vehicle and always drive in convoy with a lead driver not going faster than 5 mph over the speed limit. Make sure to leave some tip for the drivers.