There are seasonal roads downriver and supposedly really far northeast of the detroit area not sure where thoguh. The one downriver is called Heiss road but all it is is soem mud holes thoguh from what I hear.

Also if you go here and look at teh venues part ont eh left it shows some of the places to go
http://www.glfwda.org


or you can check out the link in my sig for trail rides to these and other places like Canada or the Badlands offroad park in indiana for instance.