Here is a picture of the differential on my dad's 1983SD, I am confused as to which plugs must be removed to fill/drain the fluid....I was thinking that "B" is the upper plug, and that "C" is the lower drain plug....am I right? I got confused because I saw the foreign object "A" up at the top and I don't know what that is. So which ones do I use? This diff is begining to whine and is a 3.07 with almost 258k on it....probably neglected by previous owners as well, I don't even want to know what the fluid (or whats left of it) looks like Plus as you can see in the picture the whole bottom half is coated with a sludge...which I hope is not leaked out fluid Also, what size Allen Socket is this? 12mm? or 13mm? I only have a 10 and I can see that its quite a bit too small....

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