I looked at my blazer and through my '74 and '86 service manuals and could not find anything that looked like those. I have a stud on the frame the rod hooks around and the rest of the brackets I saw are much heavier.
They look like they should hold the connectors for the tail lights. Mine does not have them. Maybe that is why my tail lights would collect snow and ice and unplug themselves. I can't find a picture in the service manual, but I'm thinking they go into the holes on the top of the frame. One of the holes is for the filled panel bracket.
You could try Keith Seymore too. He might recognize them.
