normaly the tires are the limiting factor, the other parts are enginered to have a fairly large saftey margin built into them, for example the rear end that is in my truck is actualy rated at twice the weight in a different brand of truck than it is in my ford, brakes are not usal anything to worry about when towing as anything that heavy has its own brakes. automatic transmitions don't care about the actual weight but rather how you drive it to get their, they are rated for more power than the truck can put out. now you might have to do services a bit more to keep the fluid looking nice and maybe install a cooler if it is tending to get warm onthe highway but thats it generaly. standard transmitions on the other hand are a different story, haha.
to be honest I don't think you can get a tire rated high enough to worry about other parts within reason. if you go to comercial 22" tires maybe but that isn't reasonable. to me the higher rated tires are just to eliminate the only weak link that is around thoes capacities and reduce the chances of blowouts.