In the most broad of senses, I would say that one level of differentiation between the two is that the whole truth might not be one of tragic proportions. I think that the idea of tragedy is one that can be different from the whole truth. Tragedy involves some level of transformation, some movement from one point to its opposite. For example, Homer's depiction of Hector as one who is poised between equally desirable ends, but ultimately incompatible ones is tragic because Hector can never win and must choose a path that he knows lacks totality. Flaubert's heroine, Emma Bovary, is one poised in the tragic condition because her desire to appropriate thr world in accordance to her own subjectivity ends up proving to be socially disastrous. These situations are tragic because the characters move in a circle of pain. The "whole truth" might not be as tragic because it does not involve that level of drama and irony combined in pitting an individual in a condition of pain. The "whole truth" might simply be a situation that is difficult.