I did look up the barcode on the old rear diff that was on the rear cover and that seems to match up for the 3.91 rear differential - I am assuming that the internals are the different for the 3.91 despite the pumpkin itself having a identifier that is for the 3.46 or 4.44.

The rear diff I was sold was for a F15 series which while it does bolt up to the car, the splines don't lock in - in fact you can see a gap on one side where the output shaft has maybe a cm of daylight between the dust shield and the pumpkin itself signifying that the pumpkin itself is too small. And there is so much play that there is no way that would work to deliver power to the wheels without it just tearing itself apart again.

It is actually getting a little difficult to find these 3.91 rear diffs that aren't for other vehicles as the one major difference I see in them are these spacers or whatever on each output shaft side that then bolt into what I assume are the output shafts themselves which means it would need different output shafts as they bolt completely different than just the splined version of them. Of course there are a plethora of M rear differentials, which wouldn't work again with this setup without changing out the output shafts along with whatever else.