I'm now on my 2nd set on the back. I have 20 inch tires on my 4.8 and I am a little miffed with the tire wear on the back. The front now have about 40,000 km on them and I expect that they will last another 10,000 km. The first set of rears only lasted about 20,000 km which seemed low to me.
Since I am from Canada I purchased my tires from Tire Warehouse ( in the US) and got them shipped. As a previous poster indicated these tires are hard to find and not very available in Canada. In fact when I put the 2nd set of rears on there were no 20 inchers available in Canada.