Don't be too hard on yourself. That is one of the most common problems, honestly we should've caught it. Your newb terminology and the problem with the spider gears did throw us off though. At least you have a greater knowledge of your vehicle.

I'd consider replacing that bulkhead though. The stripped screws could allow the mesh of the diff gears to change and chew them up. I'm sure the bulkhead is cheaper to reace than the ring and pinion gears.