You should change your rear differential fluid every 20-25k miles. Depending on how harsh you drive your car. To check to see if your low, just unbolt the bolt on the back of the differential case and stick your little finger in there. If you can get fluid on your finger, theres enough in there. To change the fluid, just unbolt some of the bolts at the base of the differential case and let it all drain out. Then, just fill it back up through the bolt on the upper-rear portion of the differential cover. Typically, it will require 3qts and a 4oz. bottle of friction modifier. Be sure to use synthetic gear lubricant as well. Motorcraft makes some in a 75w-140 weight.