When I first got my M818 5 ton, it would rock back and forth at speed. After reading a few threads here about it, I decided to move some wheels/tires around to different positions.
I can't seem to find it now, but somewhere I read that one should attempt to get the average of the combined rollout of the tires of each of the rear axles to be as close as possible. The differential will take care of any left to right variance, but there is no such differential between the rear axles, so if the tallest tires are on one axle and the shortest tires are on the other axle, the tall one will want to to "pass" the other one.
The shortest tires go together on one side of an axle with the tallest on the opposite side. The remaining tires go on the other axle in such a way to have the average rollout between the two axles as close as possible.
After I arranged the tires like this, my truck stopped its "bucking." This process is easier on a M923A1 because there are 4 less tires to deal with.