If you scan for diagnostic codes and non are returned, I would start by inspecting the drive train.
What's the mileage on the car?
What's the state of the front differential bushing? When it fails it usually results in a clunk upon downshifting or WOT downshifting.
Upon failing the items below usually cause a vibration not a clunk per say but worth checking anyway while under there just in case.
- The rubber flex disc couplers front and back of the drive-shaft, look for cracking and tearing at the bolt locations.
- The drive-shaft carrier bushing and bearing, inspect the rubber condition.
Also this might be a good time to check the diff fluid and change it, if the old fluid is 8 years old in there might be a good time for it.
Those are the easiest things to check first. If all of the above look OK without cracking or disintegration, only then I would turn my attention to the trans and engine.
Speaking of trans and engine, it's not a bad idea to change the trans fluid and filter if that never was done before and the car is at 100K+
And if the car is high on miles and still on the original plugs, it's a good time to change the plugs and inspect the coils and coil boots.
And concerning the fuel filter, well it's not like the engine was starved of fuel, then it would have most probably stalled or something. I change mine every 60K miles, however the fuel filter in the CTS can easily last much longer before needing a change.