Tardy update, but some info that might be helpful for others.
Turns out the pinion was trashed. Worn to the point where the pinion gear teeth are sharpened like blades. Very sharp. No apparent damage to the ring gear.
A local guru replaced the R&P, rebuilt the LSD and rear axle setup - including rear wheel bearings and more. Said it was "all messed up" so it's hard to say why this happened. It does not appear to a Mobil1 failure - more likely the R&P was misaligned, although not sure why that happened. At least one rear axle had splines twisted since it was backing out of the axle.
Lemme know if you have questions or wanna see pictures, and thank you for the helpful advice.