I've been listening to some Jazz music as I work here, and one of Ella Fitzgerald's songs had a little riff on "Plymouth Rock landed on them". It stopped me working because the riff seemed so cheerful I couldn't reconcile it with the Malcom X version (which is very full of angst and outrage). Still, don't see any other major occurences of the phrase, so I suppose it must be a Malcom X reference... Ella's voice is just too warm and cheerful sounding to render it harshly and critically.