I'm gonna guess the crush sleeve crushed some more, let the pinion slop around and ate it up. If you decide to put it back together, put a solid crush sleeve eliminator in the case. Way better set up.
You need to take it all the way down to clean it up. Inspect the gears CLOSELY! I have a rear that did the same thing, and cracked ring gear teeth almost the entire outer circumference.
The sure grip, or posi as you put, is rebuildable. There are kits ranging from $10 to $45 They all do the same thing.
If you didn't pay too much for it, and the parts are good, it's a nice starting point for a rear.