To further complicate the issue, you do not need a Discover Pass to drive roads that are open to street vehicles, just to park on state managed recreation land.
Also you do not need a Discover Pass on any vehicle registered as an ORV anywhere that ORV use is legal. I have dual registered my tow vehicle and have not had any problems yet.
That is the letter of the law.
Although the courts have upheld the second scenario, don't be shocked if a DNR law enforcement officer cites you for a Discover Pass violation anyway.