Rotating the rear axle means to rotate the axle housing on its own axis, so that the shaft of the pinion gear is pointed more upward, toward the putput shaft of the transfer case.

In a vehicle with a leaf spring suspension, such as a CJ or a YJ, the rear axle is rotated by installing tapered shims between the springs and the spring pads on the axle housing.

In a vehicle with a linked suspension, such as a TJ, the rear axle is rotated by changing the relative length of the upper and lower control arms. However, unless either the upper arms or the lower arms are adjustable, that's not an option. Most "long arm" lift kits for vehicles such as the TJ contain adjustable upper control arms.