Two things that can help with the troubleshooting:
1- Jack up the rear end and turn the wheel by hand, see if you can notice any resistance or noise.
2- More dangerous, jack up all 4 corners then put the car in drive, check any rubbing or noise.
If your axle is dry and dirt got into it, change the whole thing, I am sure that there is a lot of them available used as they rarely fail. I don't even bother taking them from parts cars...
'85 Euro 635csi Race Turbo, megasquirt, Group A
'92 525i Touring 5 speed, 3.46 diff, UUC 8.5lb flywheel, soon to be turbo.
'02 Subaru WRX Wagon with STI springs, bigger sway bars