THE Dixie Chicks' biggest detractor, FreeRepublic.com founder Jim Robinson, declined to appear in their documentary, "Shut Up and Sing." Now Ken Sunshine, who reps the George Bush-bashing Chicks, calls him "a coward. A real man would put his money where his mouth is, but this Robinson guy, he'd rather hide behind his keyboard than have a real discussion about his ideas . . . he's just a chicken[bleep] fraud." Robinson's rep, Kristinn Taylor, confirmed he ducked the interview: "He told me to go ahead and do it."