I took my F 150 to a local mechanic to get the differential cover and gasket removed, the fluid drained and then everything put back together. I didn't want to bother. My mistake. I got the truck back, put a piece of cardboard under the rear differential and determined that it was leaking slowly. I went back to the mechanic, they supposedly fixed it, but the cardboard check failed again. I went back to the mechanic and they told me it was the pinion seal bearing that was leaking. This problem never existed until I had taken the truck to them. I got the truck back in the pinion seal was no longer leaking but I still had a slow leak from the gasket around the differential cover. $400 later in three trips back to the mechanic I gave up, bought the SPELAB cast aluminum cover and the lube locker gasket plus the necessary fluid and did it myself. No leaks since then. I particularly like the little sight glass since I was able to get the fluid exactly to the halfway mark within the sight glass. I should never have gone to the local mechanic in the first place.