Does every character need to have their own unique syntax and way of speaking?

Instead of working to make everybody sound different (a tough thing to do in print, anyway, and easily out of control), make each person unique in other ways. I once wrote a piece called Dialogue Is War, wherein I emphasized that even if everybody there has the same mission, they are all different people with different needs and wants and talents and vices. I once had twins as main characters. One was raised in Italy and the other in Ohio. One was super competitive as well as jealous, while the other was a mediator. You need to not have generic people. They all have different ideas about how to achieve even the same objective, and different paths to get there. This will all shade what they say and how they say it. With that in hand, you will likely not need to MAKE UP difference in speech because speech follows personality.