Here you can see the floor sagging. The beam is behind it keeping the other side of the floor perfectly level. :)
This shows it a little better. You can see that the two sections are joined here. See the dark siding on the left and the lighter siding on the right? The section on the left is sagging. In the middle of the house, where we had some heavy items like a cast iron bathtub and tools, it was sagging more - close to a couple inches.
This was going to be a pain. I finally figured out I could jack it up with tow jacks on the rim joist near the sag and lift it enough to install the level pony wall.
I also went ahead and jacked up the interior and leveled the subfloor as best as I could. Eventually what I need to do is install new floor joists that span this joint. So, I'll cut out a chunk of the old rim joist, and install a new joist that spans between the new beams.