Frame flex

First why did you block the frame, you just added more weight to the pin. The axle weight is spread out over both axles starting at the from spring mount and ending at the rear mount. Do your experiment again with the axles taking the weight and see how much movement you get. I would drop any covers and inspect the frame for any damage and do your experiment with it exposed. You don't need jacks and blocks just put a 4x4 under the pin and use your landing jacks to take the weight on and off the pin. It you have any damage a good welder will be able to fix it, some movement it to be expected. I'm not defending Lippert their reputation is not the greatest but what you did was take about half of the axle weight and transfer it to the pin.
Denny