Tepponogu, sit in a few different F-150's and Navigators and see if you like the seats from those more than what you have in your truck. As long as you get the parts from the same body style truck, they will bolt in. Then comes the question of whether they will plug right in or not (too many variables to say a definitive yes or no to). But, look at the plugs and if they appear to be the same, you should be golden. Some of the King Ranch F-150's have a different seat than what the other trucks do. Kinda like the Lightnings as I recall came with a different seat. You may find one more comfortable than what you have. If I remember right, you can even grab the seats from out of a 03-07 Expy/Nav/F-150 and they will bolt into your 97-02. So, that may open up the options for you.
As for making the ride more comfortable, that is very subjective. Also, what are you willing to spend to make the ride better. The big mod that I am thinking of is converting the truck over to a true air bag suspension setup. For you, this will do 2 things. The big one is allowing the truck to "kneel" further. This will make getting in and out of the truck easier as you will not have to step up as much to get into the truck. The second thing is you will find that the air bag suspension can be made to offer a much smoother ride. So, the suspension is going to soak up the bumps a lot more. Down side to this is you may also experience a little more body roll. So, taking high speed turns may not be advisable. Keep in mind though that this will probably set you back about $3000.
LIke Freeway eludes to, stick with stock wheels if not take a step back to 16/17" rims. In short, the more sidewall you can get on a tire, the smoother the ride is going to be since you have more sidewall to flex and absorb the bumps. Unfortunately, you can not go less than 16" rims. You jump up to say 20" rims and while you still have a fair amount of sidewall, the ride difference may be detrimental to you (based on your condition). For that, all I can say is to take a ride in a friends truck that may have say 20" rims and see if that is tolerable for you.
As for other comfort things, may want to look into adding a slip on cover for the seat that has a built in heating pad (assuming your truck did not come equipped with seat heaters). This may help provide some back relief by being able to apply heat to relax the muscles.
After these options, I guess I would need an idea of what you are contemplating. Do not let the "I have never seen this in a truck before" stop you. I'm sure the guys around here can tell you that if you can dream it, I will figure out a way to make it work.