Install limiting straps on the axles to keep them from drooping too far.
Maybe, just maybe something could be welded to the shackle or to the pivot to keep it from dropping too far.
A longer shackle or a longer pivot would prevent this from happening as well; using a longer shackle would lower the trailer ride height by half of the added length (a shackle 2 inches longer would drop the trailer ride height by 1").
After my thinking while typing all this, I would try for a longer pivot first. This would move cause the shackle to be more vertical with a load on the suspension, and less likely to turn horizontal when an axle droops (or rides over a curb).
