Wick,
The upper fill plug is there is to make you put oil in and then have it run all over when you realize it drains so slowly into the pumpkin that it'll take a week to get it all in. I always fill it into the middle one and have it just below this plug. Some guys put a grease zerk in there, but I don't know why because the rearend oil gets drug up there via the ring gear. I have never had a muncie, but does it have a drain plug there or is there a crack/hole that isn't factory?