Is my ex-wife allowed to spread untrue rumors about me and post rumors on social media?

It would only be slander if it was an untrue statement of fact, not opinion.  Thus, if your ex said that you were a jerk, that would not be slander; if she falsely said that you had defaulted in paying child support, that could be slander.  Depending on what the untrue statement was, however, you may be required to prove that you suffered actual monetary damages from the slander (for example, if you were fired because your boss believed the false statement) in order to win yur case.