HTTP POST request can not support JSONP.
Here is some resource :
1) Cross-domain AJAX with JSONP
http://devlog.info/2010/03/10/cross-domain-ajax/
Then the next limitation — and this is a big one — is that JSONP doesn’t support POST requests.
2) JSONP in Jquery.POST()
http://forum.jquery.com/topic/jsonp-in-jquery-post
3) Using PUT/POST/DELETE with JSONP and jQuery
http://stackoverflow.com/questions/5345493/using-put-post-delete-with-jsonp-and-jquery
4) Ext.util.JSONP POST possible?
http://www.sencha.com/forum/showthread.php?125847-Ext.util.JSONP-POST-possible
So It need change the REST service from POST to GET. but Get has message size limitation. Another way is user server side proxy or other solution .