have you had a wheel alignment done recently?? mine had a bad vibration at 40mph (60ish kph) and was fixed instantly with a wheel alignment

Out-of-round tires or rims.
I suspect some side to side wobble as well as O-R diameter.
0.125" max wheel O.R. was a number I recall, don't bet on that one though, I'm not sure. Can't recall the side to side figure at all.

Had the same exact issue with my 75 V-8 4-sp Monza, I HAD to go over the US Wide 55mph to run smooth..

A Bud had a tire "Trueing" machine, trued up the tires, No Vibration anywhere.

Drive-shaft, {Ute}? ARE WE still Talking 1200's Here ?
If so, U-Joint 'phasing' isn't as big a deal as the independent rear suspension cars unless jacked up. Still I feel the tires/wheels are the issue.
Why, cause it goes away at more speed, the "Out of Balance" drive-shafts got worse with speed.

Still make sure they're "opposed" U-know, one flops up and the other to the side?

On the 510 and the like, the rear Half-Shafts must be opposed or the car will jump up and down at walking speeds.

yep all definately firing

will try some of the things suggested
thanks

yeah my one does it too, certain rpm 4th gear between 60 - 80, kind of annoying but i try to ignore it.

Try running it up to speed on jack stands with the wheels off. be sure to put wheel nuts on the studs to prevent the drums/discs from moving.
If the vibration is still there, you know its the tailshaft.

Wheels are usually close to 50-60kmh, drive shaft higher. Old holden drive shafts will vibrate at about 95kmh if they're out. The solution is to get it rebalanced or get another one.

i have the same problem here,maybe i might try to rotate the driveshaft.

Under braking??? So it goes when accelerating but at idle it shakes?

Are all 4 cyls firing?

It just vibrates the hole car
It goes away when i accelarate?

Is the vibration through the steering wheel or just felt in your feet etc? Mine was only through feet. Steering wheel and gear stick were fine.

Also could be I found my drivers ball joint fully seated after 3000kms and I had to do the nut up tighter