doing a brake job, after removing both rear diff i noticed on the left side inside the gearbox..where the diff shaft goes into the transmission I noticed what looks like a gear that goes around the shaft, then the shaft goes farther in the transmission. There is not one on the right side.. not sure what this is I know it goes on the shaft, but what is it's purpose? it's loose, just sitting in the gear oil, free turning and moves around.. stumped