Can we make Ajax request to a different instance of server(different port) running on the same machine
If you use easyXSS (http://easyxss.net) and follow the example at http://xsslocal.kinsey.no/projects/easyxss/example/methods.html, then you could do this quite easily by having a document on the domain that the xhr-request are hitting doing the real requests.
With some trickery. Try JSONP.
Different port means different domain for the browser. So you will hit the cross-domain barrier.
Like Stuart said, you could try using JSONP.
If you are using jQuery, I'd recommend http://code.google.com/p/jquery-jsonp/ if you want it to be as painless as possible.