I know there have been a few threads about this sort of thing recently, but maybe someone can point me in the right direction on this one...

My KJ GTV has a shudder through the whole car that is "sometimes" there at around 100kph, sometimes not. For example, I drove 300k's yesterday & every time I was in the 95-105kph range it was present - and quite bad. This morning, having decided that I was going to take it to have the suspension checked, no sign of it at all in 40k's.

It can't be wheel balance(otherwise it would be there all the time) and it doesn't come through the steering wheel, so I guess it's not the rack.

I have just replaced the lower suspension ball joints at the front and the front shocks are about 75k old. When I did the ball joints, the control arm bushes looked OK, & all suspension nuts / bolts are tight.

Oh yes, and the wheel nuts are correctly tightened, unlike in the recent thread!

Any ideas what to look at?

Wheel balance is most likely the culprit.
As a double check at 100 kph or when the shudder occurs use the tippy and try 3th and 3rd gears.
If the shudder is still there then it is not a driveline/ engine problem.

Personally I would have all 4 wheels rebalanced. as you say the shudder doesn't come through the steering wheel it would indicate that the problem is at the rear of the car.
Gerry

could be your cvs, had the same problem with my te going at 100-110. it orccured at 2700 rpm. changed the cvs (think it was the drivers side) and problem gone. my front wheel bearing were also gone and changed them at the same time so that could have contributed.

Yeah, my CV joints are SHAGGED and I have a pretty knarly vibration at 100-110 kays an hour.

does it go away when you take your foot of the accelerator? if so its usually the transmission needs service, cv's, or engine mount. if its still present even when coasting its usually wheels/wheel balance/wheels bearing

Your problem could be electrical if it only occurs sometimes as your post states especially if your car does not have high miles on it.

Defective tyre with a bubble in the sidewall. Front or rear.
Wheel alignment out.
Loose steering component(s) - tie rods, tie rod ends, ball joints. (were they installed correctly, torque wrench used?)
Worn wheel bearings.
Rear suspension problems putting rear wheel alignment out.
Drive shafts out of balance, partly seizing. (cold it's ok, warmed up you get vibration)
Run out on brake discs, pads touching the high spots at speed. (especially if there is some play in the wheel bearings)