71 swb posi rear end problems

[Brently]

i have a popping in the rear end when i slow down and turn - one explained it like i was hopping over a brick - a hard pop but only happens when i slow way down to turn - i have been on here before and it was suggested to change the fluid in the rear end before doing anything else - my questions now are what type of exact fluid do i need to use - is there anything that needs to be added because it is a posi rear end? also how do i identify the exact gear ratio of a rear end - i am rear end ignorant - thanks, Brent