You will definitely want to consult with an attorney regarding this matter. Your employer may have a qualified privilege to your claim. An employer has a qualified privilege to make a statements about the activities of its employees arising out of your employment and your on job performance. However, if you can prove that your employer made the statement with actual malice, your employer will lose that qualified privilege. A qualified privilege will defeat an otherwise viable employer defamation claim.
Actually, such statements could be defamatory per se...meaning they are presumed to cause damage. Biggest issue is to decide whether you want to spend the money on a lawyer to prosecute the issue.