it's about metatheory--philosophy, epistemology, ontology, etc. it's NOT about substantive theory. it's the difference between marx's writings on human nature vs. his historical sociology of the French Rev. or his work in Das Kapital. it's not symptomatic of modernity to search for a normative ground since that's what philosophers have been doing for centuries, yes? in fact, many would argue that postmodernism and poststructructuralism have rejected any possibility of locating a normative ground upon which to engage in social theory and empirical research.
like i said to carrol, marxists have typically provided a normative ground in a standpoint theory: the working class, the intellectual vanguard, organic intellectuals, marginalized intellectuals (horkheimer and adorno), marcuse's great refusal (brain farting on what he calls these), materialist feminists once argued that women would be the source of 'truth', and black marxists have argued for that the social location of blacks as the most revolutionary (and, therefore, possessing the 'truth').