I am not sure this is an issue or something normal.

My KJ2 has a slight humming vibration around 1,700-1,900rpm.
It goes away (but not completely) the warmer the car gets.
The more load, the worse it is, so in a high gear at those revs it's most pronounced.

It sort of feels like what you'd expect a sports exhaust to feel like (ie, the slight vibration around the drone rpm).

Engine mounts seem fine visually at least

My KJ2 has a slight humming vibration around 1,700-1,900rpm.

Is it manual or auto?
Does it do it in every gear at that RPM
or is it road speed ie. when you get that RPM in top gear?

It is automatic (5 speed).
It is not speed related, the vibration is there in any gear, but the more throttle pressed and more load, the more evident the vibration is.
As mentioned it is worst when the car is warming up, and after about 20 mins driving it has subsided considerably but never quite goes away.

Having not being told this I assume most passengers never notice anything

OK. To put it simply, it's eng related and it's heat related.
I wonder if the exhaust touching the body somewhere could be ruled out?
With you saying that it sounds like the drone of some exhausts, if it touches the body , that sound is amplified . Of course.
Might be somewhere to start.

The exhaust isn't touching as there is no sound.
I was relating the vibration to that of an exhaust drone vibration (ie, high frequency buzzing/humming vibration).
It could be normal engine vibration, I am not sure how smooth the 6G74 is meant to be, but I feel maybe it should be smoother.
For example I would have expected it to be considerably smoother than a friends VX 3.8L Commodore, yet it's not really any smoother at all

Without sound stupid here, it couldn't just be a misfire under load and therefore be your leads/plugs could it? I mean bad leads do cause a vibration when misfiring, and would stay there even when engines warm.

Damn near impossible to diagnose via web forum. It could be any one of a thousand parts vibrating as the engine RPM hits the right harmonic. About the only thing I can suggest is to try to narrow down the source, which will be tricky if it only happens under load. If you can't reproduce the vibe in your driveway by gradually going through the rev range and listening carefully you'll probably have to put up with it until it gets worse or goes away on its own. If you're concerned best bet is to take it to a pro.

No it's definitely not a misfire

It is a difficult diagnosis.
Is it possible to post a vid or audio of the sound.
Windows up and the radio off of course. :)

Verandah, there is no sound. It is just a slight humming type vibration that you have to pay attention to notice. I don't know if this is common to all 6G74s or not.
I am not sure how smooth the 6G74 is supposed to be, but on the reviews from autospeed etc it's praised for it's smoothness so I'd expect it to be smooth.

Is it possible that my exhaust supports have gone hard?
Muffler causing issues etc?

It might be a loose heat shield on the exhaust somewhere.
Just loose enough to vibrate at a resonant freq.

One of the heat shields actually is dented quite a bit. Next service I will have a look at it