Body flexing issues with 2001 tahoe

[Modded]

24s could have something to do with it too, they are so heavy and it's also almost a 10 year old truck....

Definitely not the 24s, they are not any where close to heavy enough on these SUVs, for what it counts, 28s would not be eavy enough to cause that.
I would say a body mount issue like Eagle said.