The two terms are about as synonymous as “Karl Marx” and “Groucho Marx.” Carl L. Bankston III tells us more about the cultural dimensions and historical developments involved:
The idea of “social justice” stems from cultural perspectives shaped by two developments of the late twentieth century: the rise of mass consumption and the heritage of the civil rights movement. The former fueled a growing preoccupation with wealth distribution, and the latter provided an energizing myth that animated the redistributive ethic.
With so many utopian, redistributionist thinkers in and out of government promoting “social justice,” you’d think they—or, at least, somebody—would have a clear definition in mind of just what they’re pushing for; not exactly:
The term social justice comes up frequently in circles concerned with political and economic policy. Although it is often ill defined, it generally rests on two overriding principles. First, social justice is viewed primarily as a matter of redistributing goods and resources to improve the situations of the disadvantaged. Second, this redistribution is not presented as a matter of compassion or national interest, but as a matter of the rights of the relatively disadvantaged to make claims on the rest of the society. In common usage, the term is rarely taken as expressing a debatable position, but as a statement of a fundamental axiom of value in political and economic life. [Emphasis added]
And there is much—really, too much—dependence on myth-making in public policy disputes:
Although social myths are useful for creating solidarity and motivation, they do pose problems as ways of understanding the human environment. First, myths obscure the differences among events and situations. Seeing disabilities or variations in sexuality according to the civil rights model minimizes the differences between these aspects of the human condition and the historical experience of African Americans. Second, social myths oversimplify our institutions and relations by dramatizing them as sharply drawn stories of good and evil. This moral simplification not only reduces social theory to caricature but also invests political positions (such as redistributive arguments) with automatic virtue. Third, a widely accepted social myth imposes a template on thinking that discourages alternative views or the examination of assumptions. In particular, the civil rights myth discourages approaching questions of political economy as matters of competing interest groups or as matters of overall national interest. Instead, it answers these kinds of questions with assertions of irreducible rights.
Eventually, though, achieving “social justice” always comes down to the use and—too often—the abuse of governmental power:
The most troubling assumption in both the perspective and the theory of social justice involves power. If justice is a matter of organizing society in the best interests of the least advantaged, then the quest for justice necessitates unending efforts to reorganize society in the name of those interests. A society, however, is not a specific institutional entity or even a set of procedures, like a legal system. A society is the total sum of interactions and historically shaped patterns of interactions among people. The goal of reorganizing society as a whole, then, is essentially a goal of reshaping how people choose to live and think. This goal is implicitly totalitarian, although it certainly does not necessarily lead to totalitarianism because of the many real-world barriers to translating moral goals into political action.
The introduction to Bankston’s full article (“Social Justice: Cultural Origins of a Perspective and a Theory”) in The Independent Review is here (PDF, 464 KB, 14 pages).