Hey I mean this in the most constructively critical way possible but I think there are some more fundamental problems in your serve that you need to address before the leg kick back issues. The lower body issues are moot if you can't effectively transfer the energy from your upper body into the ball. I'll try to give you some of my personal insights that I think will set you on the path towards a more sound serve.
One thing that you should work on(like the poster above me mentioned) is getting a deeper shoulder/hip turn. In the trophy pose, in my opinion, you want your racquet head to be at least little to the left of the line of your body. Right now it looks as if your racquet head is too far to the right before you start your upswing into the ball.
Another thing that I feel would be useful is working on getting a good solid 'throwing motion' into the ball. It seems as if now it's a little bit like you're just laying your wrist back and swinging at the ball. Think about how (no offense intended) girls throws vs how guys throw. At the moment your motion is a little too close to the former. It's tough to describe but right after your trophy pose during the 'racquet drop' phase of the motion you really want to lax your wrist/forehand (elbow kinda tilts upwards and racquet points down at the ground), this will allow you to really accelerate your racquet head into the ball.
Good to hear you made the grip change to continental, that is fundamental before all these other things can be put into place. I'd really work on these two things at the moment and I think it will make a tremendous difference in your serve if you practice with this in mind over the next few months.