Hi there,
WE got a 94 Discovery, 3.9V8i, done 200km on clock. lol

When you get to 80km per hour, the gear shifts into the last gear/top gear (4 or 5), (automatic) and we get a shudder/vibration. You can accelerate out of it, and also never does it on any other gear. Only when you wanting to cruise at 8okms.

Currently out looking at main Drive shaft with the rubber donut on it: looks ok, front drive/prop shaft: "U" joints look ok, no give in it when seeing if it loose.
Transmission mount?
Fuel pump?
And hopefully not gearbox/transmission - this would not be good

Any other ideas as to what it is?

thanks

I had exactly the same with mine (D2 4.0) and it was the coil packs and leads breaking down. The torque converter locks at 50mph ish (80kmh) and loads the engine . I bet if you put it on a diagnostic it will say multiple misfires. new leads and coil packs sorted it.
Andy