How about just `_jsonp` or `_jsonpParam`?  For setting name "[jsonp](http://api.jquery.com/jQuery.ajax/)" the jquery api states:
> Override the callback function name in a jsonp request. This value will be used instead of 'callback' in the 'callback=?' part of the query string in the url. So {jsonp:'onJSONPLoad'} would result in 'onJSONPLoad=?' passed to the server ....

So using `_jsonp`  would be inline with the setting name used in jquery.

For  setting name `jsonpCallback` the jquery API says:
> Specify the callback function name for a JSONP request. This value will be used instead of the random name automatically generated by jQuery...

So using param  name `_jsonCallback` in our code would in fact cause the confusion we are actually trying to avoid.