After noticing the rear "grinding when cold" begin several months ago, I was in no rush to resolve only because how many rear ends have failed here? None....
This past w/e I replace the rear with 2 new qts of Non-synthetic since it was on my shelf, along with the 4oz bottle of "friction modifier" as recommended by Ford.
I felt the slighted rumble the 1st time I drove it afterwards and that was the last I've felt it. I've driven it from cold, 4 times including coming to work this morning 4:30....no grinding after backing out of my driveway.
I will warn you though, while the drain was relatively easy for a lefty to loosen, the fill took an hour to get the proper length cheater to finally break it loose.
jeff