Dave, If this was an operational problem we manufacture and sell a unit known as the TXP it prevents drivers from engaging the opposite gear until such time as the speed is brought to zero and their RPM is back down have a look and call us if you want a quote. FMS Technology in south africa you can find our details on the web
I forgot to mention the problem has been with same two operators, It might be an operation issue such as, changing directions at speed without braking.
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.
Three different trucks, axles twisted, 2 rights 1 left