Why Can't They All Just Get Along...

I am employed by an organization that has been established for over 35 years.  When I was hired a little over a year ago, it was the first introduction of an HR position for the company.  It was an entirely new industry for me.  Not only did I have to start an HR department from scratch, but I had to familiarize myself with the industry in general and the staff.  I will be the first to say that it is a constant learning experience.  I have enjoyed the challenge thus far, but there is an issue within one department that I cannot get a firm grasp on, and I would appreciate any feedback/suggestions.

I am realist so I am not expecting a utopian culture.  I am simply expecting one of mutual respect, professionalism, and teamwork.  Basically, I want this department composed of 26 people to act like the adults that they are (ages range from mid-20s to mid-60s).  However, they backstab, gossip, and throw each other under the bus with every given opportunity.  There is no camaraderie.  I have tried everything I can think of to improve the culture… surveys, posting positive/motivational phrases around the office to reinforce desired behavior, meetings, working side-by-side with them personally… to no avail.  I, of course, do not see this behavior.  I only hear about it.  The “pot stirrers” and situations change as frequently as the weather and even then I only have he/she said, he/she said basis to go off of.  Some days I feel like I am monitoring a group of high school students that can’t get along.  Has anyone experienced a similar situation, and what did you do to rectify it?