Mine's had a light clunk in it since the day I bought it in Feb 07'. Apparently lots of JK's suffer from it. It could be like mentioned above, something worn out, could be a loose trackbar bolt (torque it TIGHT, mine was kinda loose from factory), or it could be related to the actual steering box/shaft like this thread is going on about:
http://www.jk-forum.com/showthread.php?t=11927&highlight=steering+clunkYou could also try adjusting your steering box, but I'm a little unsure how to do that, Nathan gave me the quick explanation but I don't think you'd want to play with that unless you know what you're doing. I doubt it'd be unit bearings if it's still relatively stock. I've had 35's on mine for about 10 months now and they still have no play. The 07-08's also had pretty crappy ball-joints, so it could be that too; mine are okay-ish, but could be more snug. Mopar makes a better replacement model now if that's the case.