Unfortunately, if you do not know the actual identity of the person posting the defamatory information about you, then a lawyer would have to: (1) hire a private investigator to try to befriend the defendant, and thereby discover his or her contact information; or (2) file a defamation lawsuit against the defendant as a "John Doe" (anonymous) defendant, and then subpoena the internet service providers to try to obtain the defendant's identifying information.
This could be a very expensive process. I suggest that the private investigator route would be less expensive than the immediate lawsuit route.
Regardless, based upon your stated facts, you are not ready to hire a lawyer to send a cease and desist. There is some groundwork to be accomplished, first.
I realize that my answer may not be exactly what you were hoping to read. However, under the circumstances, the best that I can do is to explain what the law is and is not, so that you can avoid expending valuable resources looking for answers that do not exist, and concentrate on the options that are actually available.
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