Finding examples of "unity and diversity" in the sense that we use these terms today is difficult when looking at the foundational history of the United States. Many of the Founders were, of course, slaveholders, and most saw no place for Native Americans in the nation's future. One striking way in which the United States, as established by its Constitution, did advocate for diversity was its early commitment to religious freedom. The Constitution banned religious tests as a precondition for holding office, and the state of Virginia led the way in establishing what amounted to total religious freedom. This principle was further established in the First Amendment. Yet anti-Catholicism would be rampant in response to waves of Irish immigration in the antebellum period, and most Americans, including many of the founding generation, believed the nation to be an Anglo-Saxon Protestant one. So even this commitment to diversity was severely limited. As for unity, the nation's founders perceived a great deal of divergent interests, especially between regions, and the political unity they sought to establish was based on a delicate balance between state and federal power. Their success was only confirmed by a bloody civil war in the 1860s.
As for today, Americans are generally far more accepting of racial and ethnic diversity than at any time in our nation's history. While old attitudes, including racial attitudes, persist in many ways, they do not have the same legal sanction they did at the nation's founding. Diversity in the workplace and on college campuses, for example, is actually mandated by law and as a matter of policy. This includes gender diversity. Legal, if not de facto discrimination, is, in short, against the law. Diversity, very much unlike earlier in our nation's history, is seen as a value to be upheld, even if it means different things to different people. As for unity, commentators frequently remark on the extent of political division today, but national unity is promoted by an emphasis on ceremony (singing the national anthem, for example) that was in its infancy in the founding period.