Transmission fluid and Differential oil change

[DacK44]

I have a 1994 Automatic Subaru Legacy Wagon.
Im looking through my owners manual to change my Transmission fluid.
The tranny fluid is dark brown and Drive doesnt always engage right away when i put it in gear. So I look up how to change it.
Then i look up to change differential oil at the same time while im down there.
According to my manual it looks like center diff oil is told by the same dipstick? but doesnt the differential use gear oil rather than transmission fluid? the rear differential clearly shows how to check and replace the oil but i'm confused about the center diff and transmission fluid.
also, is there a transmission fluid filter i should replace? is it as accessible as an oil filter?
help and thanks