The plug on the diff cover has never opened, it is jammed in there really good and will not come out (Solid diff cover). So we have always used the one on the front of the diff and I do believe it is much higher. So it may be overfilled. But it has always been that way and even shops that I have taken it to has filled it the same way to the same depth.
Well, I just did a little googling and found a couple conflicting results.
1. The way it is supposed to be filled is to open both ports (the high one on the front by the pinion and the one on the cover), fill it through the one by the pinion and it is full when it starts to come out of the hole in the cover. It should hold about two quarts. (But I think this may be for the IRS setup without the axle tubes in the equation).
2. On Jeepforum, it says that it will hold a full gallon and with the Solid Diff cover, even more. But then had issues with it coming out of the breather line later.
So still not clear on how much should be in it. But kind of sure that even if it leaks the way it is, it isn't going to cause any damage. But I will take a look at it later today and see if I can do anything about it.