Generically speaking, SAS takes care of the imbalance for you. But, if you give us the research question you are trying to test and more details on your design you may get useful input. For example, do you know that Diagnosis is not used in the model structure of this GLIMMIX model outside of distinguishing among IDs across different diagnoses? Your model has a g-matrix with N*N dimensions. Diagnosis is not part of your model and, therefore, your balancing question is not relevant.
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