I had the same problems with pain in my wrist and it does have to do with relaxation. For years I was taught to prepare but never to relex. Perhaps looser wrists? A tense wrist will cause tension to wrise from the wrist to the arm and all the way to the shoulder. I once injured myself terribly because of tension.
My father (not a pianist or any kind of musician) would always say the following about playing piano: "It is all in the wrists."
And you know, I think he may be right.
I think your father's right to a point; loose wrists (or more correctly the ability to loosen them instantly as needed) are essential for a healthy piano technique. I've always had loose wrists, shoulder tension was my issue; it didn't injure or cripple me but it certainly hindered my technical command and ability. The other day I found an old videotape of myself playing jazz in a club 16 years ago; I played well, but how? My shoulders were jacked up to heaven. But after some time of experimentation without a teacher I arrived at a technique that works for me and I believe can work for anyone.
Robert, your tension seems centered in your forearms more than your wrists, although they are so close together that it's hard to tell which is first, like the chicken and the egg. Either way you should remedy this situation as it will lead to carpal tunnel syndrome or tendonitis. You are particularly prone to injury imo because of your highly developed and fluent finger technique. Your fingers are doing most of the the work in your playing, and your tense wrists are literally choking your hand and finger tendons; this will inevitably lead to CTS (carpal tunnel syndrome). Glad that you're already making progress with this. I would do the gravity drops that Chang and many teachers reccomend, that will make you start your playing from a relaxed state as opposed to a tense one. Cydonia's trick is familiar to me, I do that to my students to make sure they are relaxed; without warning I will move in and check their wrists and/or shoulders to make sure there is no extraneous tension. It's the only way to really know what's going on there.
I don't think the cure will be difficult for you, Robert. You are a good pianist and a sensitive musician and are working to correct the problem. There may be a nessacary period of adjustment as your technique changes from angular to fluid (or actualy a combination of the two). Pick your repertoire carefully and with a mind for working these problems out and obtaining the relaxed state that you desire.