verify that there's nothing caught in the drum.
then, pull the diff cover and inspect. could be a busted spider gear or carrier. if it is, you'll likely need to source a used axle as there are NO repro parts for any of the various 9 3/8s designs.
i would suggest not running the driveline til you get this figured out. posi rear ends aren't designed to 'free wheel' one side or the other for extended periods of time. if it really IS a posi, you're tearing up the clutch plates by doing this.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)