If it's 2WD....EASY!!! Romove the driveshaft.(some fluid may come out). To romove the oil seal from the end of the extention housing, they do make a special tool, but if you use a hammer and screwdriver(carefully) you could just pull it out. Inspect the contact surface of the seal for damage, use a seal installer or a REALLY BIG SOCKET to drive it back on straight. Reinstall the driveshaft and you're done.