I"m not very good with putting my thoughts into words, more like my experience here lately. I just noticed this a few days ago, when turning on curves, it feels like the rear end is "rolling", Floating, i don't know how to explain it any better.
Luckily I have only 28k miles on it. I am going to take it to the dealer ASAP, because frankly i feel like my truck is either going to fishtail or rollover.
Keynotes: 8ft bed, 4wd, 4.3 liter.

I would like some feedback from others if you're familiar with this or may know what I'm talking about, and yes the tires still have 65% tread on them.

 

Only things I could think of are that the rear of that truck is so light, that it just swings out from underneath itself or the rear u-bolts are tightened all the way down. If that doesn't fix it then the dealer needs to check it out or you could buy a rear sway bar kit and see what that does