According to Alldata, the rear on my 2005 has a total capacity of 3.2qt. It also says that the oil level should be within 5mm of the lower surface of the opening, so if you filled it to that point I would think it's fine.
And one more thing...... the rear diff was under a slight vacuum or positive pressure when i removed the fill plug. It was like PSSSST.
Aren't diffs vented to atmosphere?