Again, run the forum posts and the emails by an attorney for review, to see if there is any action you can take against the person who posted them and impersonated you.
Bringing a defamation action against the online poster could be extremely costly and, therefore, must be a factor to consider carefully. In other words, it may very well not be worth your time, your effort and your money to legally pursue the matter.
Whether or not filing suit against the poster makes sense could depend on how much reputational injury you have suffered as a result of the posts. There could be an issue about the "obscene" content of the material posted, and this content could cause problems for you if you are thought to have posted it - so have your attorney check Texas and federal online obscenity laws.
Any threats of violence that seem like serious threats that put you in fear for your safety should be reported to the police.