Nicholson Creek would be the place I'd go. Marias Creek to Nicholson is pretty much roadless. You have old skidder roads with Kelly humps, so pickups won't be buzzing all around you.
Be advised. ORV's are 100 % LEGAL to ride in the Okanogan National Forest if they are registered with the correct, up to date stickers from the dol. They don't have to be street legal either.