Dave, Do any of these trucks have evidence of being hit on the drive wheels (same wheel as twisted axle)? If so, axle housing spindle area could be slightly bent causing axle shaft to constantly flex.
Are tires true pneumatics or Solid pneumatics? Or are they driven on rough surfaces (pot holes)? How about wet or slippery surfaces? Tires could be breaking traction over holes or wet and then the sudden "bite" on dry surface puts extreme stress on axle shaft.