I've dealt with this issue so many times I lost track. When tires, shocks, springs and everything else has been examined and eliminated, it's time to start looking at a REAR sway bar. At least an inch in diameter. Adding a rear sway bar is flat-out "magic" to cure sway, leaning in curves, and instability in cross winds. You will not believe it is the same vehicle!
Google ADDCO SWAY BARS
They are located in Lake Park, FL.
The installation of a rear sway bar DOES NOT MAKE THE RIDE STIFFER! If you attemped to cure sway with stiffer springs, your rig wil ride like a buckboard.
I "discovered" ADDCO when I was attempting to tame the handling of a 400hp Sunbeam Tiger, 40-years ago. The kits are not cheap but nothing else does it nearly aa well.