Rear axle "tow" rating not GAWR

Do Chrysler engineers use a formula or a value or rating for determining the shear or tensile stress on our rear axle or 5th wheel hitch? I know that there isn't a practical way of users to determine what it is but...Everything that is talked about with specs is just load or vertical load? If I have a 17000# toy hauler fully loaded and have a 1400# plus sandrail behind the 3 axles then my pin weight might be within the 6000# GAWR of the truck (6000#- rear axle truck dry scale weight) because of the location of the sandrail load but what about the pull or tow weight of the load/toy hauler? Or do you assume that my 6000#GAWR vertical load of the truck is the weak factor. In other words, can the internals of the rear axle or be over worked with a horizontal load before you hit the GAWR of 6000#. My father in law fried a bearing in his rear axle pulling a heavy toy hauler recently. Yes, he was over loaded and never had it on the scales but I am curious why horizontal load is never discussed or is it worked into the vertical load specs? Or am I just missing something??