That's the pinion seal on the rear differential. Not difficult to replace... but something i'd recommend doing with a friend that has some experience.

To replace that seal you need to take off the yoke which also loads the pinion bearings. So before you remove the bolt you need to use a beam style torque wrench to measure the rotational torque; then when you re-install everything you tighten the nut until you match the same rotational torque. This makes sure you don't over-tighten the pinion bearings and seize them up.