Okay, this is an easy one. When your jabbing, and moving forward, or WHENEVER your moving forward, you always want to push off of the back foot. Dont ever "step forward, then drag the back foot". Stay on the balls of your feet and try pushing off of your back foot first to move forward, and push off with your front foot first to move backwards. You are correct in that you want to jab as you are coming toward the end of your step. It should all be in one fluid motion, and thats why it feels natural for you to do that. When your pushing off the ball of your back foot, you get more energy into the jab. Im not saying to "leap" into the punches. You always want to keep a strong, balanced base. Try the footwork without the jab first. Working forward for 15 feet, and then backwards 15 feet. Keep working that a few rounds until you become more comfortable with it, and then start to add your jab into it while your moving forward, and backward.