[eluser]MRosenello[/eluser]
Okay, so I finally figured this out and it was not anything to do with codeigniter. Rather, it had to do with NTLM authentication and IE. When a user was first visiting the site I authenticated them with NTLM then stored their LAN ID and Domain in a session.
For some reason doing it this way messed with the headers that were being sent. I changed it so that the user is reauthenticated every time they go to a page that will post data back to the server. Once I did this it started working as expected.
This is also the reason why it worked in every browser except for IE. This is supposedly a known issue/feature with IE.
Again, thanks for all the help.
-Mike