Land Rover Repair: p38 rr shaking from 50mph to 85mph, factory wheels, rear diff

Question
hi i have a p38 2.5 turbo diesel, ever since i have owned it it has been shaking at motorway speeds, sometimes it will be through the steering but mostly through the body and can be felt worst in the back, it has 20 inch stormer wheels which i have had balanced loads of times and it makes no difference so the shake. Is it that the wheels are too big or is it something more serious like a UJ or rear diff on its way out?
Thank you in advance for your time.

Answer
Even the factory wheels often had problems due to the wheels not being centered on the hub.  The factory used shims to align each wheel on the center of the hub.  It does sound to me like your tires or flat spotting or there are flat spots on the rims.  Flat spots on the rims will not go away no matter how many times you balance them.  

I would suggest you take it to a specialized Land Rover repair shop and se if they can put on some factory wheels and take it for a drive to see if this fixes the problem.  If it doesn't they can take a look at the driveline.