Chock the wheels, apply the park brake, get a big pry bar, drop the driveshaft, install the correct socket and breaker bar, and give it all you got to get the pinion nut off. Knock up the edge of the seal, grab it with pliers and fight with it until it comes out. Gently tap the new one in and reassemble. Torque the pinion nut to spec. , reinstall the driveshaft. Check the diff. lube to make sure you haven't packed the diff. full of mud. Hopefully you won't need an impact or a 3/4 drive.