Kevin, I no longer have that differential, but in this case I think I'd loosen the lock nuts and tighten all three of those bolts. Then I'd watch to see if the play disappeared. There will be some room between the bearing and the shoulder...I would see if those beveled bolts would cause the piece to pull forward to move the pinion bearing away. If that doesn't help, then obviously something is allowing the pinion gear to travel toward the rear...where deceleration is going to cause it to want to travel anyhow.
