Yep i'd be more inclined to suspect bushes or steering components and sadly i'm having similar issues with vibration over 90kph, immediately suspected the center bearing support which although was starting to tear wasn't the source of the vibes. I have a full set of front and rear bushes and a TRW track rod kit so that will be my task for the weekend.
I fitted a new center bearing anyway while i was under there.

 

Exactly like going over a rumble strip.
We have had the car since 23,000 miles it is now 214k we have transmission fluids changed .Last batch from Atlantic British and the first time I ordered fluid from auto shop in area making a point to get the correct stuff.
Could the Control arms be at fault..or center front bushings.They have done water line replacement and we go over this miserable road every day for like a year it may have damaged front end ? I'm going to have him keep car and check suspension parts. How much is a transmission rebuild in this car would you know ?

 

Please tell us if this solves issue.Im going to have him check front end components.And put the anti shutter fluid in if he finds no front end flaw. We did the front bearings Timkin maybe year and a half two years ago but this terrible road we take home may have ruined something

 

Inspecting it today i found a split boot on the front right driveshaft so i think that may be my issue.Got a good shaft from a wrecker cheap so will swap them out tomorrow and see how it goes.

 

I spent a lot of time and money on different things when I had a vibration above 65 mph on my Discovery 1, it ended up being overfilled tires. Check the easy stuff first.

 

Check all the usual areas, tyres, etc. But I once developed a vibration at speed but only sporadically. Felt like a misfiring engine. Developed to a clang while driving. Suspected center prop shaft bearing going, or gone, but it was rear prop shaft CV binding at the rear diff at certain point of rotation. Replaced the prop shaft and all is good.

 

This feels like going over the ridges they put in the road to warn you are going out of lane.

 

Changed the shaft but still got the vibrations but not as bad, it feels like the whole can is shaking which is making it hard to diagnose.
Bushes,bearing and track rod/tierod on the RH side are tight so need to have a better look at the left side.

 

DISCO3.CO.UK - View topic - Juddering at 70
This is on the UK Forum for LR3 (D3)
relating to an engine misfire that feels like a shuddering Torque Converter
a few posts down from the top titled vibration at 70 mph

 

She says going up hill on highway it sometimes make a bit of a catch or grab to next level instead of smooth.