You'll need 3 qts. I really really doubt you got the 9.25". I used Royal Purple Synthetic in mine, cause I get it the cheapest. It works awesome.
Use some Ultra Black or Ultra Grey, don't try anything blue, won't stand up to gear oil very long. I made this mistake (used ultra blue cause I had it sittin around the casa) So now I get to dump out expensive oil, and do it all over agian!
I personally ran some 85W140 in mine, cause it was hummin a bit (think it's wearing out) but I'd run 75W90 unless theres somthing wrong with your diff.
The Rubber plug is easy, just pop it out once the cover is off! put a thin bead of UltraBlack around the plug when you reinstall it, should curtail (SP) any leakage there! (My plug doesn't leak, just the edge haha)