On the front of the pinion housing (area at the front of the differential where the driveshaft connects to the pinion flange) there is a plug with a 3/8" square recess in it that a ratchet should fit in. The plug is on the passenger side of the housing, remove that plug (with the vehicle on level ground) you can put anything in the hole to see oil is near that fill hole. Sometimes they are overfilled and oil will come out as soon as the plug is removed, so don't have your head under it. If the level is way below the hole you will need to put gear oil in and that should be 85/90 wt gear oil. If the level is less than half an inch below the fill hole then you don't need to add any oil. Once filled replace the drain plug. If you had to add oil then you should also look for places that the oil might be leaking from. Good luck.