edit: I guess it's now starting to be safe to use the native JSON.stringify() method, supported by most browsers (yes, even IE8+ if you're wondering).
As simple as:
JSON.stringify(yourData)
You should encode you data in JSON before sending it, you can't just send an object like this as POST data.
I recommand using the jQuery json plugin to do so. You can then use something like this in jQuery:
$.post(_saveDeviceUrl, {
data : $.toJSON(postData)
}, function(response){
//Process your response here
}
);