Change Select value using jQuery Uniform.js
After setting the value as usual, you need to tell uniform to update the element.
$.uniform.update("#myUpdatedSelect");
or as the documentation says, if you are lazy you can let it update ALL elements
$.uniform.update();
Try to use the update command in the complete function:
$('.selectbox').load('index.php?get=newoptions', function() {
$.uniform.update('selectbox');
});