Rough idle in 2008 CTS
 

My 2008 CTS with DI v6 engine runs rough at idle. I am getting a P0302 engine code also, which is cylinder 2 misfire detected. The suggested fixes are replacing ignition coil, spark plug,or fuel injector, but the engine has good acceleration and runs good above idle. So, I'm thinking there's a low probability any of those fixes would cure the rough idle. I saw in one thread concerning rough idle that there was an ECM update that cured this problem. Is this something only a Dealer has access to? And has anyone done this recently? Or could this be a vacuum leak problem somewhere?