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The torque converter is balanced on its own. Unbolting it from the flexplate will mostly eliminate it as a source of vibration. Part of it will remain in contact with the crankshaft pilot hole so it may continue to spin even though it's been disconnected from the engine.

The harmonic balancer is basically a rubber bushing between the crankshaft and pulleys. The rubber should deflect 'opposite but in harmony' with vibrations, hence the name. Shaft moves left, rubber compresses on the left and expands on the right, then as the shaft moves right, the rubber expands on the left and compresses on the right (over-simplified but should get the point across) and should result in a net-zero movement in the pulley (think of the air-soft BB in the tires thing). If the balancer has been contaminated by, or otherwise exposed to oil or coolant, this can affect how the balancer reacts.

Exhaust resonance... well... can't say it's not the problem but can't say it is either. You're not running a highly tuned, performance oriented engine, so I'd likely brush that one off.

Oddly though, no one mentioned engine and transmission mounts....
They are what connect the driveline to the frame and fatigue in any of them can result in odd vibrations.
Considering you are running a relatively large displacement, long stroke, 4 cylinder engine, they tend to vibrate enough on their own. Maybe a reason porsche and others have put counter-balance shafts in the engine design?

 

There's also that anti harmonic damper thing at the rear of the tranny/transfer case.

 

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You give up too easy.