No offense taken. It's just that under 4-5k the truck accelerates, stops, handles, and rides very close to no trailer at all. These were not boat/trailers though. Just trailers, uhauls, snowmobile, landscaper's, even a dolly with a wrangler some 1200 miles.
When your over 5000 everything changes acceleration, stopping and the trailer will jerk/pull the truck when you go over bumps, so it "tells you" it's back there.
At 8k+ everything changes again. It's hard to get going, the trailer pushes the truck at stops till the trailer brakes take hold, you can even feel the trailer pushing the rear axle sideways in the turns (tail wagging the dog).
I guess the old rule of 80% of towing capacity comes from this, and one i try to follow.