1992 Toyota pick-up rear differential oil

I have a 1992 toyota sr5 pick-up with the extended cab. I had to replace the rrear differential gasket and now I need to refill the differential with oil the problem is I can't get my fill plug out because it is to tight and won't budge. I was wondering if any one knew the amount of oil the rear differential takes so I can fill it from the vent plug without being able to check the oil level?