Assuming that the x and zRotation of the BODY are zero when you yRotate it, the only thing I can think of that would cause it to 'lean', is if the orientations of the BODY are not zero. With the BODY selected, open the Joint Editor, and check that all the orientation dials are set to zero.

Also check the 'Joint Order', as this can affect how the BODY yRotates when the x and zRotation of the BODY are not zero. Normally the rotation order for the BODY should be YXZ.