I was under my 46 1/2 ton this weekend looking at the differential and realized there was no bolt or fitting on the bottom to drain the oil. I have the shop manual & real axel overhal manual, but neither mention how to drain the oil. Was it designed that you removed the back cover to drain? Or is the lower torque tube lock nut supposed to be used as the drain hole. Sorry for the newbie question (I tried the search function first). Thanks.