70s/80s Toyotas/Nissan mini trucks were available with dually rear axles (GAWR of 4,400lbs) and 1 tonne ratings. I believe the U-haul put a 10' cargo box on them, not sure if frame rails were extended by Toyota or the company doing the conversion for U-haul.
S-10s of the early 80s could be optioned with short ton payloads.
Flex in an SUV is different than in a pickup:
a. rear seat passengers get a lot more concerned than a load of dirt; bodywork of SUV is less tolerant of flex than a pickup box.
b. SUV tend to be heavier on average than pickups - most of us don't load our truck to GVWR on regular basis.
So, manufacturers stiffen SUV frames more than pickup, which makes them heavier.
p.s. Don't confuse strength with stiffness, while they often do together, making something to stiff can actually make it weaker.
From my treasure chest of useless information:
Ford 9" with large (Torino) wheel bearings and 31 spline axles has GAWR of 3,600lbs. The more common ones with small bearings and 28 splines axles only 2,900lbs.
Ford 7.5 and 8.8 in Rangers with their 28 spline axles have 2,750lb rating, Explorer (and FX4) Rangers with 31 splines have 3,200lb rating and the 8.8HD in my '17 F-150 has a 3,800lb rating with 31 spline axles.
I have seen rating up to 5,300lbs for the 9.75"; Sterling 10.5" has rating up to 6,250lbs SRW. Supposedly, there is a 8,250 DRW 10.5" found in some US gov't vehicles/early '90 C&Cs, but I think its like sasquatch and unicorns - mythical creature.