I did. I had the rear wheels up on ramps, the front wheels on the ground. But looking at it, I don't think a slight overfill would be a big deal. Any extra will just find its way out the breather tube when it warms up and expands. And yes, that's how I knew it was full, it began dribbling in my face. 54, you would think I would have learned before now not to fill a diff with your face directly under the diff. Well, I know it now!