How much water is it getting? It should still be getting the equivalent of an inch of rain per week. If it hasn't been raining, then you need to soak it, if it has been raining, could it be too wet (which I think is HARD to do to willows!)?

I would dig down next to the root ball, at least 4 inches and see what the ground is like for moisture. IF it is dry, WATER it well, and continue to monitor it and water as needed. If the soil is oozing with water, then the tree needs better drainage - but it may have grown enough that digging it up and replanting it higher in the ground may not be an option. If any of the branches are still pretty healthy looking, I think I would try to take some cuttings - willows root pretty easily, and this way, you would have an understudy waiting in the wings, as it were, should worse come to worst.

I bet since we've been in a drought you aren't getting much rain on that tree. Also they have a lot of roots so if you were digging around the tree that could be your problem.