Packet sniffers generally do not capture localhost traffic. I need to inspect some post data in a localhost environment (being generated from a Ruby on Rails development). Do you know of any programs that expose localhost packets?
I use fiddler on my Windows box for http sniffing. Since its only looking at http traffic you don't get nearly the amount of noise you get with something like WireShark.
The trick to getting it to work with data sent and received locally is to use a different endpoint for your urls. Using http://127.0.0.1./YourServiceName instead of http://localhost/YourServiceName has always worked for me. Its important to include the trailing 'dot' in the IP address. Don't ask me why though.