I agree with the others, don't go back to that shop. Those rear ends are nearly bullet proof. If it works ok, it probably doesn't need much except seals and gaskets. The only issue is the ratio which is not great for highway use.
As Bill stated the best and easiest way to go is the stock overdrive trans (SM319). Some electrical components are required but it is a bolt in. It's the best kept secret for keeping these trucks on the road.
Fred
