are you talking about your rear outpu shaft seal.. on yuor rear differential?? if so. no it's not a hard thing to replace..
I just did mine about 3 wks ago.. just make sure you have a 1/2 inch torque wrench.. and an impact wrench(important) otherwise, it'll be very hard to remove the lock nut.. it's usually Jammed in good.. hehe. once the lock nut is off. pull the yolk out.. pop the seal out clean everything thoroughly, then replace the seal with the new one.. and put your yolk back in screw the locknut and tighter it to the torque specs in a Jeep manual.. I can't remember of the top of my head what it was.
Also, while your there, check your yolk.. once the yolk is off, check and see if it's smooth or it has small lines cut into it.
this could also be the problem.
sometimes the seal cuts small grooves into your yolk which can also cause the leak..
hope it helps