Technically, you'd even want a new front shaft with a 2.5" lift if you run shocks that will allow for too much droop. As for the rear, yes, 3" and above and your CV boots will be in a constant state of pinch and they will fail sooner than later. In other words, you won't need a new rear shaft right away but, you will want to plan for one. At the FAQ link states, you will NEED adjustable rear upper arms to set your pinion angle.