"I am an athlete and notice that during my baseball, tennis and golf swings, my body always pulls a little at the end of the swing, causing me to hook the ball and not get as much power in my swing. Sometimes muscles in either my neck or my low back will spasm after the swing. What does this tell you?"
It tells me that your nervous system is misfiring and that your structure or frame is possibly out of alignment. More than likely you have a gait fault and/or a scoliosis ( see videos back pain and on scoliosis). This can be corrected, returning to your optimum game performance.