i have a rear diff off an 87 Z28,,i changed the fluid and gasket and filled it up. I then noticed i had fluid pouring out of the bottom..when i looked i noticed a small hole which i later discovered was the vent tube hole..Went to the dealer and got a vent. It is a small round plastic tube with a white cap on one end,,

i am assuming the end without the cap is the end that you insert into the hole on the diff..problem is it seems to be a really tight fir,,like you would need to pound it in with a rubber mallet or something..is that suppose to be like that? i took pictures but i can not post them,,,thanx

 

fluid pouring out of the bottom? that sounds like a different issue. as for the vent peice, try tapping it in with a hammer. best thing to do is put a small peice of wood over it (if possible) as not to damage it. usually they are hard to press in, and i dont think ive ever seen a screw in one