Did you solve this problem??? My Disco4 is doing the same thing! Thanks!

 

I have a D4. I had the same problem, after trying 3 sets of brake rotors, new tyres and control arm bushings, the issue was with one of the front wheel bearings.
It felt fine but I noticed fine rust gathering around the axle nut. I dont believe the axle nut was correctly tightened from the factory. Took a gamble and replaced the bearing/hub after careful reassembly remiving the rust then a coating of copper grease and vibration over 80kmh (50mph) has gone away!
The rust was only on the left side (RHD Disco) and after pulling the axle spline, everything was as dry as could be with fine powder surface rust no grease ever! The excess rust around the hub housing suggested that the axle was never seated correctly into the hub. This somehow caused the vibration under braking at speed.

 

Nice fix! Congrats!

 

@gtispeeder - That was not an OEM hub that was pulled. So if there was an issue with it, it appears someone replaced it for some reason at some point. Aftermarket hubs have an incredibly high premature failure rate. The other issue is all the fake Timken hubs on the market too - easy to buy a "Timken" and end up with cheap junk that does not last too.

 

Interesting... As that hub has been in the car since new from LR. The new one is definitely the OEM Timken.
In doing this job, I've now discovered a torn CV boot... Oh the joy!