I'm lost as to why you want a tandem rear axle setup on a 4x4 chassis. It sure sounds like a lot of extra engineering, what kind of weight are you planning on hauling around? I would think having both rear axles being drive axles on a car hauler would be overkill, unless your hauling huge weight, even then, just adding a tag axle would help a bunch. Are you planning on doing 4x4 off road recovery? Gene