I was referring to the front diff fluid. It fills thru the dipstick tube. Same as the transmission fluid fills thru the dipstick tube.
But true... when replacing the rear diff fluid... make sure to loosen the upper fill plug first to ensure that you can add new fluid once it has been drained.
As far as the plugs, cap, rotor and wires... good for a tune-up... but won't grenade your engine/transmission.
Congrats on your 'new' Subie.