Rio Puerco Road is still rough in spots and its sign has gone missing. TR56 is a much smoother ride.
The OMG gate is 14.5 miles from Exit 140; 16 miles from Exit 131.
Bonus hike to Fort Red Dog: From the OMG gate, take the fence road east about 2 miles to a drainage. Follow it downstream about a quarter-mile. Fort Red Dog is a distinct reddish-orange formation on the rim. You'll know it when you see it.
There's a cave (tunnel, actually) in an alcove about 200 yards SW of Red Dog. You might need these coordinates to find it: