i did a sas swap about a month ago and went with the trail gear 4" leafs front and rear.
After installing it all the rear rode like a log wagon, i couldnt get it to flex. i removed 2 leafs from each leaf pack in the rear, it made a hell of a difference the tires will almost rub on the fenders now. Only thing i dont like is now the front sits a little higher than the rear. Trail gear said this will void my warranty on the springs but i dont care its worth it for ride quality, and offroadability

 

You could try trading out some of the front leafs for rear toyota leafs. They should have less arch but still provide support to help with stability onroad. Also you could add a rear 2wd toyota spring(more arch than 4wd) into your rear pack to gain a little height and not have it as stiff as the trail gear springs.

 

she rides good now, i just dont understand why they put so many leafs in the rear leaf packs, the front has less leafs and all the weight is in the front

 

The rear needs more arch because of how the frame is shaped. On my 4runner the frame is that way, but I think the frames are similar.
They may have added extra leafs to increase the stability on road. My brother used to run S10 leafs in the rear and had really scary body roll on turns with weight in the back.

 

ya it has quite a bit more body roll now, but i kinda like it. i need to get my gears coming, its a gutless turd with 35's

 

The weight over the front axle is more consistant so it's easier to calculate how much weight and leaf characteristics are required and give a fairly consistant product regardless of the vehicle.
More leafs in the rear because the rear will experience the most variation in loading, from nearly empty to way too much... since it is a truck and you put all the weight where?
Longer leafs tend to add initial rate (and thus ride height) and shorter leafs tend to add capacity, but that also depends on the thickness of the leaf(s) in question. So maybe add in a longer leaf and remove some other shorter leaf.