Thank you for your reply.
Most websites will take down material that is clearly defamatory (as is any post from someone claiming to be you who is not) if you write the website and request that the offending posts be removed. Once of a website is on notice that it is hosting defamatory content, it can under certain circumstances be held liable for that content if it keeps it available in reckless disregard of its falsity.
Most websites have a "contact" page where you can get a hold of the webmaster to make this sort of request. If no contact info is available, you can obtain the website administrator's contact information by looking up the domain registry for the website through a domain registry database such as this one: http://www.networksolutions.com/whois/index.jsp
Simply use the provided search field and search the XXXXXX.com part of the domain (leaving out "http://"). This will give you the website registration details, which usually includes contact information.
You may also wish to seek the services of an online reputation management service, such as Reputation Defender: http://www.reputation.com/
They will do this work on your behalf and ensure that all derogatory postings about you are removed to the extent permitted by law.
A service such as reputation defender would likely do more for you than simply hiring a private investigator. However, if you are interested in finding a private investigator, this is where you would want to go to start that process: http://www.cali-pi.org/
Unfortunately, your employer is under no obligation to prevent this from occuring unless the perpetrator is a fellow employee.
Again, my number one goal is that you are satisfied with my answer. If you are, I would greatly appreciate your "accept." If you still require further clarification, I am happy to continue assisting you.