I've noticed my RRS starts juddering when it reaches 70mph. I have no others problems when driving below this speed. I've had the wheels re-balanced and that's not solved it.

Any ideas?

Just the front wheels or the whole car? Gone - 2008 Stornoway Grey RRS 2.7 TDV6
Gone - 2009 Buckingham Blue RRS 2.7 TDV6
Gone - 2010 Alaska White RRS 5.0 V8 Aspirated
Gone - 2017 Chawton White RRS 5.0 V8 Supercharged
Current - 2010 Java Black RRS 5.0 V8 Aspirated
Current- 2024 RRS P400 SE Dynamic

The whole car judders, it was the front wheels I've had balanced.

Try having all the wheels balanced. This can be very tricky to solve. My 2008 had:

Wheels balanced, tyres changed, wheels changed, disc rotors changed, steering rack changed, prop shaft changed....at that point I refused to accost it back with <6000 miles from new. Next change was the rear differential but I never found out the result. Technicians from the UK visited Oz to try to fix it.

I hope yours is simple. Gone - 2008 Stornoway Grey RRS 2.7 TDV6
Gone - 2009 Buckingham Blue RRS 2.7 TDV6
Gone - 2010 Alaska White RRS 5.0 V8 Aspirated
Gone - 2017 Chawton White RRS 5.0 V8 Supercharged
Current - 2010 Java Black RRS 5.0 V8 Aspirated
Current- 2024 RRS P400 SE Dynamic

You don't,say what mileage is on your sport, but as you have already tried, wheel balance or even 4 wheel alignment is the first route to go down. After that, if you have a higher mileage car the torque converter could be the problem. I believe it,s referred to as the cattle grid effect, because it's just like driving across one.
Do a search on here for that subject, or vibration......... Megaflush is another subject that mentions this problem 2010 RRS, TDV8
bali Blue / colour coded
side steps / flappy paddles
black perforated leather
22" overfinch
heated everything, TV,fridge ...... now with gold calipers and mud flaps !( black mud flaps that is) !!
Many years ago,,,,, 1955 series 1 landie , sometimes wish I had kept it

Do look at the vibration posts, but.. Sometimes the simplest things work.. I had same issues, LR replaced half shafts; and considered propshaft.. However, it turned out to be wheel balance! They were balanced by a specialist and the vibration went. The dealer found this by trying a different set of wheels before ordering all the prop parts suggested by LR.. Good on them. So, if possible try that too

I had same issue. It turned out to be water in one of the back wheels. Tyre guy said never ever seen that before.
This has now happened twice,
Will upload pic when phone is charged up
Doh
Steve

It was water in both back tyres! Not sure how it got in there - anyone know?

Water could get into your wheels from the airline if the garage you use hasn't drained there compressor and don't have a water trap. 2008 G4 RRSport TDV8 current
2009 TDCi 90 current
2018 Discovery 5 HSE lux current
1994 Dakar current

.....warped brake discs maybe? I was born with a photographic memory. Shame it didn't develop!

".....do you boys know what the word Nemesis means?...."

2007 2.7 TDV6 in Stornaway Grey.
EGR Blanked.
Re-mapped by Alive Tuning.

Camelrock
That was my first thought.
The first time it happened to me, I hadn't put any air in my tyres since having it and it was a gradual increase in vibration.
When I took it to the tyre place and found the water he checked his airline which had water filtration on and it was clear of water.
When the tyre was balanced again ,it was fine for a good 4months but I noticed it again and took it back, same problem.
So far it has been fine.
My car does stand during the week as I use a van for work, I was thinking it could possibly be condensation build up
I have the original tyres on and they are Pirelli scorpion
Car is in for second service and will speak to service manager about this,
A real head scratcher
Steve

Could the water be from a pressure washer? You'd have to hold it close to the tyre bead for a while, admittedly. I'm in the shed, I'll be back in the real world in a minute.

Hi chaps,
Had my car in for service and the manager was aware of water ingress in the tyres.
They have resealed the tyres and re balanced them up FOC, it might be worth anyone else havin this issue giving the dealer a call
Steve

Hi,
I have had the same problem Perelli tyres, water in 3 tyres. I'm told by my Land Rover garage it's a known problem with these type of tyres, go for different ones next time.

Hi guys, just had the same issue on my 2013 Autobiography, nearly 1 litre of water in one rear tyre only the other three were bone dry, I've never used an airline. The dealer won't commit just said they are aware of the issue?