This feature is available in Enterprise and Ultimate editions.
You can change the location of Custom Fields on the backlog form by going to the Custom Fields section in Administration, and drag/drop the custom fields in the order that you want them to appear. Please note that you cannot move them above any system fields or mix them into system fields. This will only reorder them in the custom field "section" of the details page.
- Here's a screencast showing the operation: http://screencast.com/t/zq0QSMWf
So, what you do is go to Administration > Configuration > Custom Fields, drag those fields around to get them in the order you want, then hit the Publish Changes button. It might take some experimentation to get them in the order you want.