Hy TheYeti, I assume both the rear axles are Eaton, is my assumption correct? Most of the larger differentials do not use a crush sleeve, just a lot of torque to get the bearing preload. Have you checked the brakes on the "new" rear axle to make sure they are of the same type? As to the tires, in North America 8.25X20 seems to be the smallest size easily available.
