Changing gears on a 4x4 would probably be $1500 at best probably more at a deslership.
I use to think that the rear spring was the only difference inbetween 1 tons and 3/4 tons and working at a dealership I have people say " are you sure?". Actually I think on most the difference is front and rear torque or load rating on the springs,torsion keys can vary, brake systems or sizes can be different, and most have smaller tire sizes on 1 tons- thus effectivly lowering overall gear ratio. Also u joint sizes and drive shaft size, material or rating may differ. I cant verify all of this, because every manf. is different, and you cant find every difference, unless you cross vins and look up each part for both.
You will probably have a hard time ever finding a srw truck with axle load ratings much over 6k because most E rated tires are only rated for about 3200lb for singles.
Obviosly you will do what you will, I just like being overly safe than sorry- and I don't want to incur any liability from what I recomend.