I’m not saying they upfit anything but as the vehicles share more and more components maybe the difference between light and heavy duty narrows.I am not aware of these comparisons are possible given none of the 550s / LC250s have been available. But It wouldn’t make sense for Toyota to “upfit” the light duty platform with heavy duty 300 series stuff. Why would they do that? The whole point was a lower price point.
My internet armchair opinion is that delta you see is due to the LX 600 getting lighter due to more aluminum rather than unfitting the 550 with heavier duty components. Makes no financial sense for Toyota. They sell a zillion light duty 4Runners a year. No economic reason to make them tougher.