I agree with BarneyS, it appears your receiver is twisting up. I solved that on my truck with Torklift 30k Super magnum (overkill, but wanted the flexing to stop). I have a double stinger and no flex.
I towed with flexing receiver on previous tow vehicle and then on current truck. I decided since we were planning on keeping truck for a long time to invest in a rock solid receiver. I enjoy towing now with the setup all dialed in.
I'm not saying you need to change your receiver, more of letting you know that twisting receiver can impact your towing experience.