The Ford spec is ESP-M2C154-A which is GL-5 80W90.

I use Amsoil 80W90 synthetic in my 87. Seems to work quite good as I have 240,000 on the truck & never had a problem with the differential.

I use the same oil in the manual trans & no problems with the trans either.

It's been a loooong time since I swapeed diff fluid & I cant remember if the diff has a drain plug or if I siphoned the old stuff out thru the fill hole.