Do all Chevy duramax trucks have Allison transmissions?

No, not all Chevy Duramax trucks have Allison transmissions. While Allison transmissions were commonly paired with Duramax engines, especially in heavier-duty models, General Motors has also used other automatic transmissions, notably some models from their own Hydra-Matic line. The specific transmission used depends on the model year and trim level of the truck.