What he said. The lid for the floor drain sits on those. It's just held down by gravity.
I just called the city, and they are sending a truck out to check the main sewer line to see if our blockage is on their side of the system. My fingers are crossed, but I'm not holding my breath. Even if it's not on their side, at least we'll have eliminated that from the equation, and it won't cost us anything to do it. My luck we'd spend hundreds (or thousands) that we don't have, only to find out it was a stupid blockage on the city's side that we could have had removed for free.