Axle ratio conundrum

[MY1978]

I want to change the fluid in my rear diff (even a girl can do this!!! )  as I cannot remember if, or when, it has ever been changed (I am sure it has over 43 years at some point).   I always was told it had limited slip, how do I figure out if it does or doesn't because I know that if it is limited slip, I need to use an additive.

So here is the conundrum......it was ordered with a 4.11 ratio, the RPO build list has it at a 3.40 ratio and on the diff there is a GM8 which when I look it up says that is for a 2.56 ratio.  So, how do I know WHICH I have?  There are a series of numbers like a serial number on the diff.  There is no tag attached to any bolt.
HELP!!!!!

[VileZambonie]

When you remove the cover, the ring gear will have the high and low number separated by a colon. the formula is always driven over drive:

43/14 = 3.07 (:1)

[LILT77]

With all the different ratios you mentioned the only way to know for sure what ratio you have is to remove the cover and count the ring gear and pinion teeth and divide them. Ex. 37 ring gear teeth divided by 9 pinion teeth = 4.11 ratio. Also, since you have to remove the cover anyway you will know right away if you have a limited slip or not.

[MY1978]

Thanks, the adventure starts tomorrow.

[Shifty]

Another easy way to tell (before the cover is off), is to jack the diff into the air, with transmission in neutral and brake off, and spin the right side wheel.  If both wheels turn the same direction, it's posi.