These unique D35's had no optional locking diff. The only aftermarket one common to hear of is the Torsen, and it's $500 and only in limited production runs.
I don't know what else that vibration part could be from, ... maybe a later 98+ Ranger with a smaller engine that has space there. It didn't come from an Explorer I'd bet, who knows what one from a 4.0 OHV would look like?