i have a 91xj and i did a search found out that i have a dana 30 in the front and a 35 in the back, but the front is leaking badly i need to change the seal and the back is seeping so im going to do that to and i need to know what kind of oil i think its 75w-90 does it matter what kind? and alsow the last time i ran 4w/d it wouldnt change back to 2w/d could that be from the front diff leaking??