The relevant outgroups animating Afrikaner nationalism aren't the Zulu (or Xhosa, or Tswana, etc.) as a whole, but rather the rapidly growing black working class on the one hand, and English-speaking elites on the other. Of course Afrikaner society was and had long between hideously racist, but so was the British colonial government. It was the perceived "threat" of racial integration (and the attendant economic competition) driven by English liberals that made race the primary focus of Afrikaner politics.
Of course if you mean smaller differences that weren't in part ultimately caused by proximity, there aren't any, but that's almost tautological.
Of course if you mean smaller differences that weren't in part ultimately caused by proximity, there aren't any, but that's almost tautological.