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:

WGS84
N35° 11' 34"
W107° 03' 16"

Photos and more details
here.
Roads to the OMG and Herrera Mesa
as of 04.23.2008