When you inspected the ring and pinion, did you also look closely at the spider gears - labeled in the attached image as pinion gear (not to be confused with the drive pinion gear) and side gears? By my experience with other rear ends, the spider gears are more likely to fail than the ring and pinion (usually due to a galled pin - labeled differential pinion gear shaft - from lack of lubrication, but they do break teeth, as well). If a gear tooth sheared and wedged in the differential gears, it might cause what you experienced as "lock up."
A similar thing can happen in the transmission. Many years ago, I bought a parts car from a friend that had a "locked" 3-speed manual. When I took the cover off, I could see a broken tooth on the second gear, and wiggling the yoke back and forth freed it up so that the broken tooth piece dropped out from between the gears. At that point the transmission would turn fine, but with the seriously broken gear tooth, it wasn't going to run for long....
