Rear end

The fill hole is on the passenger side above the fin about the middle (top to bottom) of the differential.
Follow the pinion shaft along the passenger side and you should see a square drive plug about 6 inches before you get to the rear cover (that 6 inch number varies from model to model).
A 3/8 socket drive should fit in it to loosen it if it's not too tight.

Once you unscrew it, you should not have any fluid come out. If you stick your pinky finger in the hole and can touch the fluid, it's fine.

This model uses gear oil and a synthetic additive.
I have the exact same model.

Good Luck