leaking gear oil out of 8 3/4 chunk

i am leaking fluid out of the 3:55 posi chunk that is in my car. it is leaking behind where the driveshaft connects to. if someone could fill me in on the right terms for these parts. lol. i was wondering if i am able to just do it myself, or do i have to get all the tolerences redone at a shop?
where would i even order this seal at? or would i need to order other stuff too? i noticed this after wondering why my car was doing a one legger burnout.