Mt. Morris (The Mounds) is a good spot that is common amongst off roaders. I also enjoy Silver Lake Sand Dunes, but the sand will destroy you vehicle!! I think there's another Dunes area near Traverse City too.
Drummond Island is usually where they hold the Jeep Jamboree each year in Michigan. I wanted to go in the past but couldn't convince the lady!! I did make it to Camp Jeep 2004 wohoo!
If there are more suggestions for wheelin' here in Michigan let me know to. I wouldn't mind getting another meet n greet together for us Michiganders, or any travelers who would like to meet up in Michigan.