A clunking sound from rear brakes? Not something I have come a cross to my knowlege, though one mans clunk is another mans click. I couldn't imagine a clunking noise being caused by worn shoes. Now if you said a grinding noise, then I would look no further. At any rate, it's definatly worth checking out. Sometimes if a drum is turned improperly on a brake laeth, it can have grooves in it that the shoes will fallow like a needle on a record player and the shoe will reach a point on the drum and snap back. This is more of a snapping noise though and not that of a clunking.