Differentiation is one of those things that sounds better on paper than in practice. With a coursebook, it's doubly difficult because very few of them are designed with this in mind. If they are divided into groups, then you ccan use different materials with the groups, or give the more advanced ones more to do, but this does mean that much of the class will be spent in these little groups. This can be counterbalanced by work where the stronger have to interact with the weaker but have to do a more complex task, but this kind of activity takes a long time to design, so it's not practical on a frequent basis.
How different are the levels within the class?