I just want to encourage you on your goal for weight loss.
There is another currently running thread with this video of Tommy Hass hitting: http://www.youtube.com/watch?v=N8wwX...ature=youtu.be
Notice how much lower he is able to sink his upper body as he winds his torso in preparation for each shot. Look especially how low he gets on the backhands.
I know that currently your knees could never take that kind of abuse.
But use tennis as a "carrot on a stick" to keep your weight loss going, so you can eventually use a great leg push off and bigger core rotation to power your strokes.