its an interesting argument, but yes and no... let me explain the best i've been told... or at least i'll try to
if you're on 'National Forest' service roads, yes you need a plate. tecnically on state DNR roads you sould be but they don't get picky about it... but if it is a logging road, no, you don't have to be street legal in either case ... just know what you're on top of & you should know if you're okay...
i've never ridden walker, so i'm not sure if you're supposed to be street legal on that particular road... it's state DNR land, so you're probly OK.