Can I sue for Libel/Defamation?

I have been the victim of harassment for about two years now from the stepmother of my daughter. In public blogs on Myspace, she has spread rumors and lies, and swore they were true. I learned to ignore that, it's not the issue here. Apparently, when I stopped reading her blogs or reacting to her rumors, she decided to take it a step further. On Monday, I was informed by my employer that they had received a letter. The letter was from a person that I have never met, and had never been involved with my employer in any way. In reading the letter, it followed the exact lines of what she had been writing for two years, and basically this person was trying to get my employer to terminate me for unfounded reasons and accusations. Instead of confronting her, I instead allowed my daughter's father read it, not telling him what it was until he had read the full 7 pages. Afterward, he admitted that it sounded like exactly something she would do. He took the letter with him, went home and confronted her about the letter. The next day, he came to me, angry at me, and I didn't know why. He informed me that she swore that she didn't write the letter etc. And there is no way to pin that particular letter to her, unless I were to get the FBI involved and get the email traced to get the IP address. But I'm sure the FBI has much better things to do... On with the story, the next morning I receive an email that was sent to the HR department of my employer, and copied to me. This email was from my daughter's stepmother, with her name, address and email address on it. No questions this time. In this letter, she first admits that the goal of the email/letter is to convince my employer to terminate my employment. Then she claims that due to the first letter, that I have threatened her, her family, and her property, which is untrue. I can honestly say that I have never even met this person, because we have never gotten along. She also claimed that I am a thief, that I distribute lewd pictures of myself to my coworkers and clients, that I am not qualified for the position that I hold, that I have had romantic relations with my supervisor, etc. This goes on for quite a while. She implys that I am a drug addict, drink and drive, just about anything that she could say to make me look bad. None of the statements in the email are true. Also attached to the email were pictures that she had stolen from her husband's email account. Photos which he was the sole recipient of, and had told me had been deleted. Needless to say these were not PG rated photos. In any case, the pictures were from when he and I were still involved, and I had been told they were gone. She hacked his email account to get them, then attached them to the email sent to HR. So, now the HR department at my work has indecent photographs of me from 3 years ago. Luckily, I have not "yet" lost my job due to this. After the first letter I had a meeting with HR, the Division Chair, and my Supervisor, and they assured me that they had checked into my record and credentials, and that I was fully qualified to hold the position. Needless to say, after two years of public humiliation, and now bringing it into the only job I have ever held where I absolutely loved my job, I am fed up. Do I have a case for Libel/Defamation? I have an entire 3 ring binder full of blogs/emails/letters/text messages that have been posted/sent to use for evidence. Also, what am I looking at as far as cost wise for a Libel/Defamation case? I am behind on bills as it is, but I have to take care of this before I lose my job. Thanks for any info you can give.