Use the DVE Shadow, and work with it's X-axis rotation, and maybe a little Z-axis to fake perspective. It works best with single actors, since its illusion is based on a single point in space, and won't match up multiple shadows without some luck.
It comes with two tabs: one to control the quality of the shadows, and the Geometrics tab to control its position and rotation.
If you don't have that feature (maybe just for XBlur-I don't know), then after pulling the key, turn that clip into a subcomp, and after rendering it, load it into the source window. In a layer below, of same length, add a color clip of your shadows color. In the source window, deselect the video tab so only "M" is depressed. Then use the "replace edit" button to copy the matte into the color clip. Then you're ready to manipulate the shadow layer with Geometrics, Composite, Convolve, etc.