Sounds more like an issue with the RIP and its driver or flattening the appearance in general. I would investigate how it actually interprets your file. I'm sure this is simply something with flipping a switch for transparency handling or sending the file in a different format to the RIP, e.g. choosing a different PDF language level.
Dude the only way to make it work is to export the file as a tif and that is the only way!!
I hate it too when i get illustrator files with drop shadows and filter effects. It never comes out the way it's supposed to!!
If they want drop shadows they should use photoshop and save the printer the headache!
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There is no need to avoid raster effects, just be sure to select an adequate resolution (300 for print)
You want your shadow (most likely black) to overprint everything under it. If your rip won't handle a PDF with transparency properly, one work around is to set the object and its shadow to multiply, duplicate the object, then remove the shadow and set it to normal (not multiply). Flatten the PDF only as a last resort.