"...The axles were replaced 15,000 miles ago with napa Chinese parts..."

No chance you still have the original axles laying around your garage, eh?

It sound to me that under certain loads, the CV is jamming up and causing a bad vibe. Then in other conditions, like changing lanes, the load is thrown a different way, freeing-up the CV.

Is it a case where, for instance, turning left always induces the vibration and turning right always smooths it out? (or vise versa)?

Make sure the center axle bolt is still good and tight otherwise the outboard end of the axle may be backing out of the hub and allowing the CV to collapse too much.

Inspect for other suspension components that may be severely worn, allowing the wheel to shift laterally, although if that was the cause I think you would be experiencing other steering issues.