Well, to quote an old song from the Big-Band era,
"The foot bone's connected to the ankle bone. . .
And the ankle bone's connected to the leg bone. . .
And the leg bone's connected to the knee bone. . ."
and so on, or something like that at least.
But you might investigate a seat cushion like one of these:
http://www.safecomputing.com/carseat_cushions.html
before getting too carried away with other potential causes.
I'm pretty sure the auto manufacturers don't put a whole lot of extra design or materials into car seats, and the drivers' seat gets used whenever the vehicle is in motion. If your truck has enough miles on it to warrant a rebuild then the stock seat springs and cushions have seen their share of wear. If an add-on cushion alleviates the pain, even a little then you know you are on the right track.