Hello from the end of the world...Chile

Your rear shock absorbers might need to be replaced if it's that stiff.
I have performance shocks on all four wheels of mine, and I find the ride quite comfortable.

we bought the truck brand new, and the ride has always been terrible, I don

I have KYB shocks.
I wonder if you truck has a different rear spring pack than ours do.
Snap a picture of yours and post it up.