I've heard about them up at Vermonster.
But on their website it says
"There are no public, legal off road trails for 4x4's in the state of New Hampshire."
NOT true AT ALL. There are over 2,000 miles of Class VI roads in the state of NH that are exactly that. Legal, open to the public, and can only be driven in a vehicle with 4wd (unless you're dumb or careless).