I'm about to change the fluid in my rear differential and would like to know how much gear oil I need? I also have a posi (g80) and was wondering if i still needed to use a limited slip additive (i think i saw on a website that gm no longer needed to use one but just making sure). I know gm recommends a 80w-90 gl-5 oil.

Oh yeah, 1995 GMC Jimmy, 4x4, 2 door, 4.3 Vortec (W) with 3.73 ratio

Yep, 80w-90 is it - if you go syn then do not use any RTV on the cover or it will leak later.

What about the limited slip additive? Needed?

80w-90 is approved for conv and limited slip so has additives.

You remember how much i need? Also, Mobil 1 makes synthetic gear lube but it only comes in 75w90 or 75w140, would it hurt to put in the 75w90?

no it will not you will be fine with that