agree with OP about the 4runner seats... however, i've added custom foam to the last 6 vehicles ive owned to make them more comfy.
i found the 4runner seats OK in the beginning, but after about 1,500 miles, it felt like i was sitting in a hole. (Im 6'2 & 240 lbs.) I then added some foam to the lower cushion and it was like night and day.
then i took a 3,000 mile road trip and my lower back was killing me (right at the belt line on my pants). it got so bad on that trip, i used a sweatshirt behind my lower back for padding. you actually can feel a horizontal bar on the seat back right where my pain was coming from.
when i got home, i went back to my favorite upholstery shop and simply had them add some foam to the lower back of the seat (i just got back from the exact same road trip and had zero issues.. i can now drive 9+ hours w/o any discomfort).
my advice: just go down to an upholstery shop in your town that is highly recommended. try and find one that does auto interior restorations and custom work, if there are classic cars there, you probably are at the right place. not to sound racists, but i have found Hispanics to have serious talent when it comes to upholstery work.. but I am located in San Diego.
the physical work itself is a fairly simple process... just make sure you try out the seat before its recovered and reinstalled. they can shave and trim the foam so its a perfect fit for your body. i think the whole process took no more than 90 mins.
good luck
p.s.- on a side note, you mentioned your seats are hard... actually its probably the soft foam padding thats making the seat frame more noticeable.. at least that was the case for me. most auto manufactures use the most minimal materials they can get away with in order to save cost. also, remember these trucks were designed by little asians and not big americans!