Well, I will answer my own questions. I lifted the forklift today and found the drain plug at the very bottom of the differential case. The fill plug is the one at the front of the differential. They are both 1/2 in square recess and can be driven with a 1/2 in extension on a ratchet.

The oil level is to be at the bottom of the fill port.. The fluid capacity of the differential on my truck is 6.4 l and adding that amount after draining the old oil brought the level up to the bottom of the fill port.

I used 75W90 Spirax by Shell.