Outkast and BIG haven't sold anywhere NEAR as many records as Pac, overall.
This "Diamond" certification is only domestic (US-only). Worldwide sales, it's been at 15million+ for YEARS, so has "All Eyez On Me".
When I went to Johnny J's studio, he had a plaque for All Eyez On Me that was for 9 million sales, and that was from the early 00's. and I believe that was a domestic one also.
Tupac has sold about 40 million records just in the US - I would guess that's for the most part, AEOM + Greatest Hits + MATW + 7 Day Theory.
In 2007, worldwide sales for Pac were at 75 million. Eminem is the only rapper above Tupac, with an estimated 86 million worldwide sales. (and kids, that is the power of movies for you. NO WAY would Eminem be at that level without '8 Mile')
The next closest hip-hop artists are the Black Eyed Peas at 56 million, and then Jay-Z with 50 million.