The only information that an employer is legally obligated to keep confidential is certain medical information.
The only way you have a case for slander is if ALL of the following are true, and the burden of proof is entirely on you.
1.) The information your co-workers are sharing is false
2.) The co-workers KNOW it to be false
3.) They are publishing the information to an audience
4.) As a direct result of this publication, you suffered damages.
If they are saying that you violated a policy, and you DID violate the policy, you have no claim of slander. If they are saying that you were fired for a policy violation, and you WERE fired for a policy violation, you have no claim of slander. Truth is an absolute defense to a slander claim.