OP, your situation is normal. I have the exact same thing, and asked the exact same question when I bought the truck. I noticed it on the freeway here in SLC. Last year I drove across country and found the exact problem on the freeway in Tulsa. It's the only other place I noticed the issue. I don't know if the cities used the same contractor, poured the cement at the same length, or what, but it is definitely a road issue and not a Raptor issue. My son purchased a TRD Pro brand new last year and has a similar problem, so I know it's the road.
The issue can be helped a little by running lower tire pressures...but it doesn't go away.
Some have said replacing the rear springs with Deavers/Icons and torque arms fixes the problem. I haven't yet done that so can't confirm one way or the other.
I can confirm that rebuilding the shocks does not fix the problem. Tried that.
I can also confirm that going from load range E KO2's back to load range D KO2's does not fix the problem.
Some trucks seem to experience the issue more than others. I have no idea why that would be. But don't feel like there's something wrong with your truck. It's fine. It's annoying, but don't spend a lot of money hunting for a problem.
The upside is you get used to it. And the truck is so much fun everywhere else, it's worth it to put up with your voice going crazy when you're singing in the truck.