so the way you describe this sounds like a spring clip that is seated in the diff carrier where the stub axle snaps in. could it be that the spring clip slipped off? or suppose the groove an the stub axle is worn?
sorry i havent had a rear diff apart, i have seen the front stub axles on 4wd automatics and the shaft on the outer end of the cv joint is held on by one of these spring clips as well
if the clip itself is worn i would imagine you would have to pop it out with a pick, and slip in a replacement.