Sorry for derailing the thread somewhat with this.
Funny, that's my background too.
I considered going back to Karate, but I remember being so bored by the stiffness of it when I was younger. Maybe you get more out of it when you're a bit older?
I probably peaked too early: earned my shodan at 10 (mega-young, I know) after studying for five years, and my sensei wouldn't progress or consider me for a higher dan until I turned 18. I continued going and concentrated on the competitive aspect of Karate, but started falling out of love with it. At 16 I stopped going, and haven't been back since.
Wonder what grade I'd have to start back at?
If you still have your registration book, you can start again at black belt if you wish.
I decided to go back to white, and frankly it's amazing how little technique I actually had first time round. Now I'm studying the art more seriously, I'm taking great pride in perfecting my kata and having good stance, two things I virtually ignored in my teens.
I also love the "stiffness" as you describe it. There's something humbling about taking instruction and adhering to the etiquette of the dojo. After a tough day at work I find it really theraputic.