The sign was in the middle of the trail with a jeep, motorcycle and an atv crossed out on each one. It was a little funny because they picked up some cantalope size rocks and neatly laid them across the trail as well for emphasis right before it initially drops into the wash.
I think it'd be best to contact the BLM Fillmore Field Office and ask them about the road. Talk with someone who knows (like the FO Manager).
I've seen similar "closures" before. One was on Road #637 in the San Rafael Swell 4-5 years ago. This road runs along the west side of a WSA, but not in the WSA. Someone had placed a WSA sign in the middle of the road and put in a line of rocks, similar to what you're describing. I had a current travel map that showed it as open. I contacted the BLM (a State Office employee) and confirmed it wasn't closed.
These "mistakes" may be done by people who just misunderstood the travel plan. Maybe sometimes it's done by activists who think they're "closing" roads by moving signs... sort of the same as those who think they are "opening" roads by removing real closed signs.
