Filling the rear Differential ???

Xs 2 on the 1/2 in drive. If it is very tight and won't go in easy, would suggest small 1/2" extension and tap it in gently with a hammer to seat it. Agree that given the age of these vehicles and the fact most of us don't know when or if ever the oil was changed you should also remove the cover and clean out well and put in new oil. If a posi you will need additive. Good luck!