My 1996 Windstar has a similar problem of air switching from panel to defrost upon acceleration. This is due to vacuum leak, which I haven't had time to troubleshoot.
Temporary fix: Since, I wanted the air to stay on panel, I unhooked the vacuum control motor (VCM) arm that controls the panel-to-defrost door. In your case, you can do this to the VCM (encirled in red on the attached pic) that controls the panel-to-floor door. This should give you air from the panel only.
Hope this helps. | 
