Most webhosts will not reveal the true identity of a poster without a court order.
If you are defamed on a website, go to court, and show proof of the defamation. The judge can issue a subpoena to the webhost, ordering that the real identity of the poster is disclosed.
The judge will not issue a subpoena or an injunction without probable cause - proof that the material is defamatory, obscene, pornographic, infringing on another's rights, or an invasion of privacy. Free speech and expression is vigorously protected.
If you want to file a defamation lawsuit against a poster and you don't know his true identity, file a "John Doe" lawsuit. Again, the court will issue a subpoena to have the identity revealed so that the lawsuit can go forward, again after you have shown that the material is defamatory.
Most ISPs will contact the poster prior to revealing his/her identity to the court, so that the poster can file a response to the court, detailing why his identity should not be revealed and presenting his own defense to an injunction or a defamation suit (truth or true opinion).
One defamation action that I am aware of went through the court system with the identity of the poster never being disclosed, even through several motions and appeals. The poster won the suit and won his right to remain anonymous.