Just to make it quick, I cheated on the composite, although, shadows really aren't that hard... you just make a nurbs plane, use the sculpt surf tool, and mold it to the shape you want, give it a use background shader, and export it as a .tga that way it keeps the alpha channel for your transparant materials and your shadow. Here is the original pic, didn't make the shadows, just modified this pic by erasing the original jeep and added my jeep.
Brian