Congrats to the boys!
But I think it's worth noting that the competition in the Rock Album category is hugely diminished since their heyday and certainly since 1972. I don't say this by way of disrespect but merely to acknowledge the huge hit that record sales have taken. I wonder if there's a way to factor in piracy or other use (from Pandora and Spotify to views on Youtube) in these polls. I wonder who would really come out on top in terms of actual ears on music.
Perhaps the congrats are really due to the Grateful Dead fans, who are honest enough to actually pay for the music that they listen to!