is there any difference between the packets being sent by Ajax, and normal packets (in attributes like source port,...)?
or how can I distinguish Ajax packets from others when I'm capturing packets with jpcap?
How Ajax sends packets? is it by an unique thread?
AJAX is just an ordinary HTTP call. In principle there is absolutely nothing special in AJAX requests compared to ordinary browser requests.
To put it even more straight: there is no difference between typing http://example.com/some/document.html in browser address bar and reading that document using AJAX. Browsers probably use the exact same code to run both requests on the low level. The same headers, same cookies, same ports...