Although I'd love the chance to make fun of or make a funny here.........
I belive he did brake and or wheel axle seal service on the right rear and when you pull the drum and axle on a full floating diff you loose gear lube in the axle hub.
Only way to fill it is to fill main diff with gear lube then jack the unit as seen here and let lube fill hub then set unit back level and refill diff.
Imo of course.