You'll get several answers and opinions on how to rotate tires, and I'll probably get flamed for this. I've always had good luck rotating 4x4 tires in an X pattern, even though it does change the rotation. I've also know a couple people that do this on 4 wheel trailers(campers) at the first signs of wear, or at 3k to 5k miles intervals. The tire brand themselves aren't ideal, but simple rotation seems to help alot. Yes, even though they aren't being moved to steering wheels that help change the wear pattern.
Most trailers seem to do more sitting than rolling too, flat spotting a tire may not be noticeable during the ride, but doesn't help things either.
Tire pressure doesn't seem to be a huge problem either, as the tread depth looks pretty even. If anything, maybe adding a couple pounds.