Did you bounce it when shocks were off?
That would give you an idea of how stiff the springs are
First defense against a stiff ride is tire pressure, tire maker may say 32psi but its YOUR tire, so try 28psi
Tires absorb the smaller bumps, over-inflated tires cause harsher "feel" to a vehicle
Springs are rated for the weight they need to support, its called "sprung weight", in cars that's pretty easy to calculate, weight on each axle plus 4 passengers spread out between the two axles
Pickup trucks are often over sprung, because they have a 1/2 ton rating for the bed, so they have springs rated for the weight of the rear of the truck PLUS 1,000 pounds more
So when bed is empty they are very stiff
Springs work by the weight they support being enough so that the spring flexes when you hit a bump, the weight of the vehicle above the spring holds the top of spring in place so bottom can go up and down absorbing the bumps
If spring has too high a weight rating then when you hit a bump spring doesn't flex/compress, it just pushes the vehicle up, so "stiff ride"
That's what can occur in the rear of a pickup truck
Shocks do not hold any weight, they are there to prevent a spring from continuing to bounce after hitting a bump
So with shocks not connected when you bounce the corner of a vehicle it should easily be pushed down and come back up, same reaction if wheel in that corner was pushed up, easily deflected up, like when you push body down
If you can't easily push down on a corner with your 150+ pound body weight then that's the ride you will have, stiff
Shocks can't change that
It cheaper for truck makers to put in a set of leaf springs rated for the full 1,000 pound extra weight
So that's what most pickups have
YOU can get lower rated leaf springs with an added overload spring to get a soother ride when bed is empty
OR
YOU can add weight to the bed until you are satisfied with the ride, although MPG will be less hauling around the extra weight, lol