Does it wilt all over, or is it just on some branches, or one side? Verticillium often affects one side or branch. To know for sure it's verticillium would need a lab diagnosis which you may not want to bother with. You could look inside a dead branch to see if there is internal staining. I would try mulching it because being raised is likely making the soil around the roots hotter.
I don't know what would do well in your area, you probably have a lot more choices than I do here in Zone 4, but one list says willows are not susceptible so what about the Japanese variegated willow (Salix integra 'Hakuro-nishiki')?
