With a 3 inch lift you might get away without needing it. When you lift it, it changes your pinion angle, and you can get some vibes at certain speeds, but a 3 inch lift should not be too bad. I would install it and test it out and see what it does. If you are getting vibes you can either get a transfer case drop, or get the sye kit and driveshaft. If you don't notice any bad vibes, you don't need to worry about it.