Let's try to put your mind a little more at ease. That piece of steel looks like a chunk of sheet metal. Does it bend like sheet metal? Most of the differential innards are hardened steel and heftier than that, so if it came from a bearing, gear, etc., it would snap in half if you try to bend it. Generally, the metal part of a seal doesn't just break off on it's own. Being pressed into the housing, it doesn't move around. This is why I'm figuring someone may have worked on the rear axle sometime in the past, and that piece is a broken off remnant of an old part (seal is likely). Me, I would leave the thing alone and keep an eye and ear on it.
