It appears Cummins still sells the 5711 ring gear!! I see it in our parts system, can call tomorrow to find out price and stock.
has 25 of the 5711 ring gears in stock. You will need to create a free account, check inventory for 5711, add one of those to a quote, and submit it for pricing. Once you get their email quote, if you are ok with the price you can call them to provide payment and shipping info.
Arkansas Valley Diesel Service also has one on their eBay store
for a eye-popping $392
A Google search for "cummins 5711 ring gear" generated a few other sources...