Unjust Termination - Legal Action?

I recently accepted a management position in Huntsville AL (an at-will employment state); it required that I move from Colorado Springs, CO. I moved to Huntsville 5 months ago and have been working hard for this company since. I was called into my bosses office yesterday (27 Apr) and summarily fired. I had never been reprimanded for anything, as a matter of fact I was told by email on Friday (22 Apr) that I was doing a great job.

Needless to say I was shocked and devastated. I was told that I was terminated because two of my four employees refuse to work for me, but this was a bold lie (I have a great relationship with three of them). There was one "problem child", but she is still there. There are employees that have been reprimanded repeatedly and they are still there. I was offered a severance package and told that I have 45 days to sign.

My question is this, do I have any recourse? Should I sign the severance package or not? Should I pursue legal action? I moved my entire life... I feel that discrimination may have been involved, but I do not know for sure. I just find it funny that they fired me based on a lie after telling me I was doing such a great job, yet other employees have been promoted and given raises shortly after being reprimanded even though no changes were made in their actions. Please help...