Help me understand sway - 6K trailer, 2001 Expedition

I would suspect tongue weight. It should be between 10% and 15% of the trailer's total weight. You won't know this unless you weight it.

Also, check wheel bearings, tires, suspension, frame to make sure all the mechanicals are OK.

Find the source of the problem before thinking of a $2500 superhitch that will mask the sway but not cure the cause.