Much of Michigan will have sandy soil and hence whoops. But there are a few spots that dont get it as bad due to locally different soil compostion. Now my southern breathern may disagree with this..but to me the more southern trails in the state will see more use, and more whoops! Head north! Also, if you can find a trail system that was groomed more recently, whoop content decreases dramatically. (eventually they will return though).
The Mio loop has some whoops, but due to more rocks not to bad at all. Good mix here - some hills, some rocks, some sand, some whoops. Some sections of the Atlanta B loop are nice in that regard as well (other parts are whooped out though). PArts of Wolverine are pretty decent and whoop free. The UP has some good stuff to offer as well. (Magoo - chime in!).
Bottom line is I dont think you can find any designated ORV trail in Michigan (particularily Lower Michigan) with NO whoops. Some are better than others. Other guys here may have particular trails they find with minimal whoop content as well.