Yep I know the VC's are sealed and that the friction modifiers are unneccesary in both ends.
If you read their site a bit you'll see Redline recommends the 75W90 for hypoid bevel ring/pinions (or maybe it was on something printed I got) hence it being suggested for the rear diff.
The 75W90NS will also work fine in the rear diff.
75W90 will also work fine in the trans.
But it is "slightly" preferable the other way round.