"Social Justice"

It's not ambiguous, it's a specific term which is largely subjective. Equivocation would generally involve talking about one and then sneaking in the other.  

There is a meaningful difference between "justice" and "social justice." "Justice" is far more broad where "social justice" has more recently been identified with politically-left-leaning issues that many would argue does not represent "justice." As for equivocation, it would depend on the context in which it was being used.

No. Justice is a broad term that typically means "fairness", or just treatment. Social justice is a branch of justice that deals with sociopolitical issues, and like any other form of justice, involves value judgements.

Equivocation involves using the same word in a different way, making for a misleading argument. For instance, 'justice' also refers to a judge.

Justice is good. 

I am justice. 

Therefore, I am good. 

Justice went from "fair treatment" to "a magistrate of law." This is equivocation as two very different senses of the same word are equated. 

Social justice is justice, by definition. But there are different ways to look at justice, so it is often differentiated in order to focus a discussion. Legal justice would refer to concepts such as people being held legally accountable for wrongdoing, or the accused having a right to a vigorous defense, or equal rates of arrest, conviction, and sentencing across demographic groups. Economic justice requires that disparate groups have equal access to the economy and equal opportunity to achieve economic success. Social justice would refer more to equal treatment of citizens by other citizens, rights to interracial or gay marriage, for example. Often debates around these issues analyze data to determine if justice is increasing or decreasing, or where it is or is not, but measuring "justice" as a broad concept is impossible, whereas we can measure equal access to higher education, for example.

These are not 100% inclusive. And the reality is that each form of justice will be weaker without justice in the other areas. But the delineation gives us a framework for discussing the issues, some commonality so that we are talking about the same thing.