The service manual doesn't show a fiber ring between between the rear bearing and the ring that the lock screws bear against. If the ring for the lock screws floats on the shaft, maybe you can push it back thru the lock screw holes to line it up when replacing the pinion in the differential housing. I have not had a differential apart, so this is just info from looking at the service manual. No idea why there was a fiber spacer in yours.
