If the ItemsSource of your ListBox is bound to a collection of items then you can use the SelectedItem property of the ListBox
private void locationsNB_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
var listbox = (ListBox)sender;
var myObject = listbox.SelectedItem as MyCustomObject;
if (myObject == null) return;
// perform your custom logic with this item
}