Actually, very few FWD differentials, or RWD/rear-engine setups, allow the diff to “share” lube with the gearbox or engine.
The simple fact is that the diff generates very, very few contaminants for its lube- typically there are no clutches or similar surfaces to wear (except in some traction-aiding differentials) there’s no combustion byproducts, very little atmospheric contamination, etc.
Thus, forcing the diff to share lube with the relatively “dirty” gearbox or engine would reduce the overall system lifespan.
Point in fact, while I’m certainly no expert, the only situation I do know of where the diff shares lube with another component, is in the older Lamborghini Countach. Which, as I recall, has a single “oil pan” for the engine, transmission and differential.
I know the Citation/Fiero drivetrain has a seperate diff lube, as does the full-size Toronado/Eldorado…
Enola, what vehicle are you referring to?