Some co-workers are back-stabbing me publicly and very unjustly, can I legally record them to prove my case?

14 months ago, I was promoted within my dept over another who had worked there 4 years. Ever since, I have been back-stabbed because some people felt she was entitled to the position, but I am the better choice. In recent times, the back-stabbing has gotten worse as another newer employee is quite the "pot-stirrer" and riles up others who don't bother to think for themselves. I intend to discuss this with our boss, who is currently on vacation, but I would really like to see if I could catch what they (about 5-6 people) have been saying about me when I am not at work. I am sick of their petty crap, its unprofessional and completely inappropriate. If I get recordings and they are as bad as I expect, I will have fuel for formal complaints. So I'm looking for info about the legality of my recordings. I have done nothing to deserve these personal attacks, nor have I ever verbalized publicly against any co-worker's character. Appreciate any helpful, knowledgeable replies!