If you don't mind sand pimples head to Silver Lake in Mich here some time. I think you may have thought about it during our last trip, right? There's usually a few of us here to head with you. They have great info at their forum site
Silver Lake Sand Dunes Forum - Index and it's a great place to spend some time vacationing. But, sorry no rocks there!
Have you ever tried to head north at all? Also, check the other forums around the area here like
Great Lakes Four Wheel Drive Association - Home or
Great Lakes Off-Road. Let us know what you come up with.