There's a number of factors that can cause/contribute to the Trol speed wobbles and this is a good list to work through...
1. Get a good balance, preferably on vehicle.
2. Wheel bearings, make sure they are tight.
3. Panhard Bushings.
4. Swivel Hub rebuild (remove shims first as a check, wobble goes with no shims then it needs new bearings).
5. Replace drag link / tie rod ends.
6. Shock bushings, or new shocks if had some punishment.
7. Steering box overhaul.
There's a school of thought that in making the handling more road friendly Nissan did something that makes the Y61/GU's more susceptible to wheel shimmy than the Y60/GQ's
If you get the proper death wobbles (steering wheel thrown out your hand they're so bad) that's a sign there's a suspension component flogged out, usually bushes.
Running larger than standard tyres can sometimes cause it too, fitting a return-to-centre steering damper can fix that.