Normal rear end behavior?

[Nixon1]

I don't remember whether it was there before or not...all I know is like I said, now it whines during 2nd gear and the Overdrive to 3rd gear downshift only... Well we took it to the place and they said they were gonna change all the fluid out and put friction modifier in. So we get it back..everything's ok.. Then we take it to the local Mustang guy, Gene here...that's practically all he works on is Mustangs..so he does a sh*tload of work to the car, and when it's there we notice that the rear end is leaking fluid because the original cover, the bottom of it is bent way out of shape. Someone tried to jack it and missed the pumpkin it looks like and snagged the cover. So it was leaking fluid...Gene replaced the cover and inspected the rear end to double check that it was really a 4.10, and he tells me that there wasn't any friction modifier in it when he changed the fluid and added some.

Drove it for around..hmm...maybe 1-2 weeks without friction modifier.