The nut is only torqued around 90lbs. so, if you chock the rear wheels front and back, AND have someone hold the outside of the rear tires while you loosen the nut, it should come off easily.
Just mark the position of the castelated nut before you start, and mark the outside edge of the diff next to the mark you placed on the nut, and count the number of exposed threads on the pinion. Then after installing the seal, tighten the nut so the marks line up AND the number of exposed threads is the same as before. The cotter pin holes should still line up. If you're using a torque wrench (which you should if you have one), the torque reading should be approx 90 foot pounds. This insures that the preload on the pinion bearing is set as before (which was hopefully correct before you started).