I'd get gear oil into the diff ASAP. I don't know if dumping it in the breather hole would be a good idea or not. I'd just pull the fill plug and fill it that way. That way you can also keep track of the exact level of the oil in the diff. The seal will probably take you an hour or two to replace, it only took like 30-45 mins per seal for my rear axle, but I didnt have to bother with CV shafts and the like either.