Since I am an attorney (and have studied First Amendment jurisprudence at length), I shall give you all this Brief Lesson in Three Paragraphs:
1. The Black Letter law is clear. Defamation is defined as "an intentional false communication that harms a person's reputation."
2. A statement of opinion is not defamatory. See Gertz v. Robert Welch, Inc., 418 U.S. 232 (1974). ("Under the First Amendment there is no such thing as a false idea. However pernicious an opinion may seem, we depend for its correction not on the conscience of judges and juries by on the competition of other ideas.")
3. Though private citizens (rather than public figures or limited public figures) need not prove "actual malice" in order to succeed on a claim of defamation, the truth (i.e., that which was published if factually true) is an absolute defense against defamation. Because defamation, by definition, is limited to false statements, true statements (written or oral) are protected by the First Amendment.