This might be a ridiculous answer but I copied my WordArt and pasted it into
Microsoft Paint. Then I used the Fill With Color option (looks like the
paint bucket on the toolbar) and clicked on the background shadow with the
color I wanted. Then I copied it back into Word.
The curious thing is that when I did this in Word 2002 the WordArt image
mysteriously disappeared when I print previewed, but in Word 2000 it's fine.
I don't know if this will help but you might give it a try.