change the url of redirection of a dropdown with auto postback

Question

Hi!! I have an .aspx page named PropertyListNew.aspx which it's url is rewriten like this:

  <if url="/Propiedades/(.+)/([0-9]+)">
    <rewrite to="/propertyListNew.aspx?currentPage=$2" />
  </if>

the thing is: I have an asp:DropDownList inside my page (with current url "/Propiedades/(.+)") with autopostbak=true and when that dropdown is clicked the postback is done but the url changes to the physical one

/propertyListNew.aspx?currentPage=$2

instead of keeping the rewriten url

/Propiedades/(.+)

what can I do????????

thanks!!! Nicole