Towpro,
I didn't see it in any of your posts.....but are you running air bags on your truck? If so they may have been overinflated which will cause undue sway.
I think that you were steered in the wrong direction by placing the 5000s on the front axle, you should have 9000XLs all the way around. I sometimes think that we should consult the experts, on this forum, before making decisions on equipment for our current and future rigs.
I've had 9000XLs on my truck for 10 years, with airbags, and they have worked beyond expectations.
Best of luck getting this sorted out.