I have looked and ran my bloody hands all over the rear axle trying to find the filler plug. The only plug I found was the drain plug and even then it seems to be sitting too high to drain all of the rear axle fluid. I could be completely wrong on this…….

 

Hello Greynomad. Welcome to FTE and to the Super Duty forum.

There is no drain plug on the Sterling 10.5 differential. There is a fill plug. You pull the rear cover to drain it. Your cover will be different, but the steps in this video are what you need to do to change the fluid.