8.75 question

I've never seen a bad case.
And the fact that one side pattern comes in, points straight to bad gears.
The idea that the pinion bearing preload is changing, points to the bearings moving, the shims hammering out, or the pinion nut coming loose. In your design, if the pinion nut keeps it's torque, then it kindof points to debris under the races, or a broken,cracked bearing support works. Which thing I have also never seen.