Nate,

August's dry weather doesn't help but given their age they should be established. Lack of watering should not be the main issue, especially when one is fine and the other is not.

My gut is telling me there is a soil and/or root issue rather than a disease one. Tree lilacs are pretty tough compared to most other shrubs or small trees. One suspect is high soil pH, which can cause yellowing of the leaves (aka iron chlorosis). However, tree lilacs should be adaptable to alkaline/high pH. So, it is possible...but I would be surprised. You can find out if your soil is too alkaline from a soil test (test kits from garden stores tend not to be accurate).

Assuming the trunk isn't hiding any physical wounds---girdling roots just underground is one possibility, especially if you bought these plants in containers. These issues can take years before they show symptoms, which mimic drought or high pH issues. 

Another odd situation I have seen before is some sort of compacted layer that is restricting the roots or pooling water around them. This too can make trees go all yellow. A water percolation test can help show if you have compacted layers in the soil. Before you dig any test hole make sure you are avoiding buried utilities.

As for the prognosis, I honestly do not know. If you start to notice dead branches/leader this coming spring, the issue could be very serious.

Best,

Shane