It's definitely a must do depending on what axle you're running. A stock D35 can barely handle it's own stock tires, let alone 35s. Mine blew out on 31s on the highway
Even a Dana 44 can show it's weakness. I would definitely recommend going the Ford 8.8 route or better. If you're installing a new axle, it's easier to just upgrade your brakes, as you're already going to be running new lines. Definitely upgrade your front axles u-joints, as those will be gaining some wear and tear.