I'm sorry,,,, I cannot
"wrap my head" around this
If you try and turn a ft tire (either one), with the hub locked, in 2WD you see the Ft axle turning (try and hold it maybe to see if you can stop it?) AND when you put it in 4WD everything's locked - right?
BUT when you need the ft axle to help you off a slick spot, the rears spin, the driveshaft spins, the ft tires just sit there.....
Makes No Sense to me, if the ft diff was "skipping" there'd be bangin & clangin
(plus you just changed the oil and, I assume, didn't see all kinds of metal or anything) I have
to believe it's a hub problem more than the diff.
Just for S&Gs I'd be tempted to jack the rear up (both sides), put it in 4WD, and give it a little
gas (DON'T GET UNDER IT!!!) to see what happens. Really ALL 4 wheels up (if you can do it) then you could look it over right! Maybe look at the ft U-Joints before the hubs for spinning without the wheel (nonlocked hub)
I donno,,,, you got a puzzler there!