It looks like the shackles are already near the extent of their range, they shouldn't be that flat.
Is the bed loaded with weight?
If it is empty, have you checked to see if the main leaf springs cracked?
With not wanting any lift, I would start with a new set of springs for your quad cab, and if you carry a lot of weigh all the time, add an additional main leaf springs, they will increase your carrying capacity about 400lbs, and wont stiffen the ride that much when empty.
Yes this is loaded. with what i carry constantly, spare tire, tools, recovery gear, bed cage, chainsaw, fuel. 350 lbs ish. And it always in there. Leafs look like this
From the front mount pictured here, they go up, to the axle, then straight back to the shackle. Definitely getting new leafs. But they’ll give me. Whole bunch of lift in the back if they held their arch. Know what I mean?