Shack troubles

[Cbray2003]

Hi all I got a problem with a shacking at about 70 80 real bad and it's not like a wheel wobble because u can't feel it through the wheel and had both front wheels balanced last week. Also u can't feel it through the gear knob that bad it's slight I've had all the engine mounts changed and there fine has anyone got any ideas on this as Im thinking maybe a drive shaft or something because it only does it under load but every now and then it shacks at 40 so lost for options now as pritty much changed all the bushes and different bits on the front trying to solve this

Any ideas be greatful cheers