Rick,
All of the XZLs do it, always on the steers. Lug tyres never like being put on the steer axle, and this heel/toe wear is how they protest, all the XZLs ive ever seen do it, the huge ones on my old bloke's MAN do it too.
Aside form rotating them fairly regularly, and keeping correct pressures in check, there isnt alot you can do. if you want the un-beatable capability of these tyres offroad, thats unfortunatly what we must put up with, other than that, go to 19.5" rims with no sidewall give, bulldozer ride, no ability to drop pressures for greater footprint, no traction offroad (can you tell im a little biased...).
Andrew