Dan, next time you are at a tire store, use your bare hands/fingers to "feel" the sidewalls of a typical ST tire compared to a LT tire. Do this on unmounted tires.
Come back and tell us what you think of this idea of ST tire "having stiffer sidewalls".
In 15", for LT, I use GoodYear Wrangler HT. I just put a set on one of my horse trailers. LR D, capacity 2100 pounds at 65psi. This trailer has 3500 lb axles, so these tires provide 17% greater load capacity than the axles.
https://www.tirerack.com/tires/tires.jsp?tireMake=Goodyear&tireModel=Wrangler+HT&sidewall=Blackwall&partnum=175R5WRHTBSL&tab=SizesEDIT: regarding sidewall stiffness, I have heard the new Endurance has a much stiffer sidewall than the outgoing Marathon. I have not yet seen a Endurance in person, so I can't say from personal experience.
