I recently purchased a John Deere 5103 and noticed that it appears there is a rod that is detached on the rear just to the right of the top bar on the three point area.
Does anyone know what this is for? Or what it is called?
There is nothing missing or detached. There should be a zerk fitting to grease it. It is for the draft control sensor as already stated.
Here is a link to your ops manual. Scroll down the left side column to the 3 point hitch section.