Can I sue a former employer and co-worker for Defamation/sSlander?

Probably not.  There is a qualified privilege at work here, because your former employer has a legal right to oppose your unemployment.  However, knowingly or recklessly false statements can sometimes not be protected, and for reliable advice, you need to discuss all of the facts of your case with a lawyer in your area.  One place to look for an attorney is our website, http://attorneypages.com

Whether or not you can sue them, you should have an opportunity to respond to what they've said, to try and get your unemployment.  Check any notices you have gotten from the unemployment agency, which should tell you about that, and follow the directions there carefully -- or have your attorney handle this, too.  If you win the unemployment case, you'll get a nice check to cover the payments that have been delayed by the case.