Thanks for the input, Jim. I'm about 90% sure it loosened up because I used the wrong torque spec to tighten it down in the first place. I got a properly large 3/4" drive torque wrench now to ensure I'm operating in the middle of the torque range of my torque wrench. I've got the same factory lock ring you're describing. What I'm thinking about doing is safety wiring the eyelettes of the lock rings together to make it impossible for them to spread apart.