I just repaired a truck for this exact concern. Everything passed, boost test, VVT test, EGR test, etc, etc. They only caught the check engine light while towing a trailer.
Called tech hotline, and engineer advised me that he suspected the variable geometry business binding up under the extra heat produced by working the engine harder with the trailer attached. I took the turbo apart and found the CHRA assembly somewhat rusty inside. I cleaned up the unison ring, and found it pitted, so I replaced the turbocharger. Again, all the testing passed, but the truck seemed to have marginally more power on the road test when I was done. We'll have to see what happens there, but I'd pull the turbo on yours and have a peek inside.