What is a concrete example of one social movement in the period after 1945 that can be considered Postmodern?
The original question had to be edited. I think that one can find many examples of post World War II social movements that emphasized change and empowerment and were representative of Postmodern thought. Post World War II social movements that increased voice did so at the cost of deconstructing commonly accepted reality. Yet, I think that the Feminist movement that emerged in the period after World War II demonstrated much of the intellectual and philosophical tenets that define Postmodern thought.
Initially, the idea of deconstruction and reconstitution of commonly believed truth is where Feminism is a social movement that held to Postmodern tenets. Feminism's primary call was to deconstruct the pathology and social institution of patriarchy. In this, the Postmodern demand to reconfigure and reexamine truth is evident. At the same time, the emergence of Feminism did much to embody the disintegration that is such a vital part of Postmodernism. Women taking to the streets, burning their bras, redefining language (For example, "herstory" as opposed to "history"), as well as transforming the workplace and the institution of government are all examples of how a social movement represented disintegration in a real manner.
I think that the debates that emerged within Feminism reflected how complex a social movement it really was. In this exploration, one sees how further Postmodernism is seen. The debates that were spawned within the movement such as whether or not women's choices were actually increased with Feminism or what defines "progress" in the movement are examples of how Postmodernism seeks to establish the only truth of change and constant fluidity in the modern setting. The intellectual and philosophical divergence in the Feminist social movement is reflective of the constant state of deconstruction intrinsic to Postmodernism.