From my experience, LR drivers only wave at the same vehicle that they're driving. When I bought my first Disco in 97, there were very few other Discos on the road, but we would all waive. When I upgraded to the 2004 Disco 2, other Disco 2 drivers would waive, but not Disco 1 or LR3 drivers. Now that I have an LR3, the Disco 1 and 2 drivers don't waive. And, of course, the Range Rover drivers have never waived at any of my Discos.
This is similar to BMWs. My dad bought a 3-series in 1985, when they first came to the US, and when pasing other BMWs the normal procedure was to flash your bright lights. Now there are so many BMWs out there that you'd burn-out your light bulbs in a month if you flashed at every BMW.
I guess the friendly traditions break down as the population grows. The same thing is true of customs and manners in general.