It would be a great help if you had some jumping pics. I have an exersize that may nelp you understand what is happening when you jump:
Start by standing straight with your feet close together shoulders back.
Bend both legs so that you are standing with your legs bent as you would while riding a horse.
Lift your left leg approx 1" off the floor.
Bend forward at the waist while maintaining the position of your right leg.
You will notice that your left leg will naturally stretch back to aid in maintaining balance.
Now repeat the same exersize but this time, push your bum back, your lower back down and push your shoulders back and together.
You should feel a major change in balance compared to #1.
Lastly, do this (minus the leg lifting bit) on the flat, and then once you feel comfortable, while jumping.