I am not an expert in this area, but I recently read a SAS Global Forum paper about using PROC GLMMIX to perform GEE modeling. The author, Kathleen Kiernan, is an expert who works in SAS Technical Support:
"Insights into Using the GLIMMIX Procedure to Model Categorical Outcomes with Random Effects"
Pages 10-11 discuss the difference between GEE models in GENMOD or GEE versus GLIMMIX. Example 3 is an example with multinomial response.
... View more