Please help, deformation problem leg movement

Right now I’m trying to solve my shapekey / deformation problem for my Elissa project…

The current problem:
When using shape keys for X axis -> it is no problem
When using shape keys for Z axis -> it is no problem
When using shape keys for X + Z axis together -> shapekeys do not work as I like them to be.

Here’s a video (swf flash file) which tries to show it:
http://home.scarlet.be/~matrix64/images/shapekeys.htm

Does any body know a solution for this deformation problem? Has anybody done this before? I know there is weight painting, but do I have use this toi get over this problem?