debug XHR in chrome browser

Start up the Developer tools (Shift+Ctrl+J on Linux) and click Enable Resource Tracking. The page will refresh. Click on the Resources button, XHRs are colored yellow. To see the post content simply click on the resource. The POST content is in the Request Headers. You can also filter out just the XHRs with the XHR button in the bar below the tabs.

Hope this helps,
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On Windows, Chrome 12, I had to right-click somewhere in the console window and tick XMLHttpRequest logging to enable this.