How To: Place a grid or graticule below the data layers in the map composition
By default, grids and graticules appear above all of the layers in a data frame on the layout. You can place a grid or graticule below all layers by converting it to a graphic element and using the Order commands on the Draw toolbar's Drawing menu.
Click Data Frame Properties > Grids and select the grid or graticule you wish to reposition.
Click Convert To Graphics. A status dialog box will inform you the grid or graticule was successfully converted into a grouped graphic element.
Click OK to dismiss the Data Frame Properties dialog box.
Select the converted grid or graticule in the layout.
Click Drawing > Order > Send to Back in the Draw toolbar.