Sheri: I have to admit it's hard to diagnose from photos. As a native species, nannyberry viburnum ought to be tolerant of local soil, climate etc. How have the plants done in the past two years? Have you seen the leaf dieback before? 

The photo of your twig looks like it could be viburnum leaf beetle (VLB) which can a problem for nannyberry viburnum plants.  Here's an article that provides lots of information including treatment options, Viburnum Leaf Beetle | Ohioline (osu.edu). The article includes photos of the beetle. Since you also write that the drying wilting leaves are also on some of nearby elderberry plants, maybe it's not VLB. 

Crown borers are species of moths that chew bark but your photos don't show evidence but here's an article with some photos so you can check.  Viburnum Crown Borers (illinois.edu) 

The brown curling leaf is not characteristic of VLB. Skeletonization of the leaf is typically seen with VLB. Have you seen this? Leaf curling can indicate a variety of issues, e.g. soil nutrient imbalance, over or underwatering as well as pest damage.

Viburnums are susceptible to several fungi and fungi-like organisms, e.g. powdery mildew. What I can see of the leaves, they don't seem to have the characteristic whitish appearance on the leaves. Powdery mildew can be a problem when the plants are in shady areas. Are your plants in sun-partial shade? There are lots of fungi and organisms that act like fungi that could be non-specific to viburnum. 

Not sure that I've been very helpful in coming up with a solution for you. You might check with a local nursery or take in a cutting from the plant.

Sheri: It is a conundrum. I've just lost a viburnum to heat and dryness -- we were gone for 2 weeks and it was too hot without enough water, but it was under stress already. 

I would do the same thing --- cut back one of the plants. If the roots are healthy, it out to send up shoots in the spring.

Thanks,

Jane