UtahJimmy said, "I would try to contact the rancher and get his permission first.
Assuming that the road is illegally posted, this is absolutely horrific advice and should NEVER be an option or even considered.
The way things work legally, is that if a trail, road, etc. has a prescriptive easement, there is NO reason to ask for permission.
Further, even if a prescriptive easement exists, and the landowners post the trail or road and people ask for permission...that can have consequences and set precedent that the road/trail is private and permission is now REQUIRED.
Then things turn to chit in a hurry and the only relief is to take the case to court and reconfirm the prescriptive easement.
Don't ever, as in NEVER, ask permission to legally access any public lands, trails, or roads.
The best thing to do is what has been suggested, and that is to contact the local sheriff and make an issue out of every one of these cases...immediately. Don't wait.
The advice of contacting WYBHA is solid...myself and the rest of the WYBHA board take public access seriously.