So, you can't feel that bounce at low speed? A neighbor had a similar experience on his 2023 Sierra at around 40-60mph and it went away after installing the new tires from 4Wheelonline plus the alignment.
Update with photos once you're done with the body lift. We're looking for a similar setup on the incoming Z71 project. It recently got 18" wheels from 4Wheelonline and new rotors and will fit 305/70 tires.
So, what size did you end up with? We just put 295/55/20 Cooper STT Pro tires, 20" fuel wheels, and a leveling kit on Sierra 2500HD. The friend mechanic said 295/60 is the widest you can fit on stock.
I'm gonna tip my neighbor about heated seat element replacement. He's looking at this neat 2014 Sierra with new brake pads and tires from 4Wheelonline along with a new bed cover. The interior is bit mess though (kids) and the driver heated seat not working.