Pretty sure that it is in the tires. I bet is you look at your rawr and then add the carry capacity if your rear tires you would come up with a number that is very close.
The 4x4's MAY (probably) have higher rated tires.
GVWR is what the truck was rated for the day it left the factory with all factory equipment.
Many many many Tundra owners modify their trucks to SAFELY handle an increased load. It doesn't do anything to change the little yellow sticker but it does impact how the truck handles. They add LT tires, add air bags, timbrens, or overload springs and just roll with it.
My old 97 F150 has P-XL tires. Aired up to 44 PSI they were rated for a fuzz under 2000 lbs each.
Thanks!
Jeremiah