Do not throw parts at problem. Two possible issues. #1, Ford had a Recall and TSB's concerning attaching rivets to various steering and suspension braces and brackets mounted to frame in the 1990's. We replaced rivets with bolts and problem corrected. It was the Loudest repair operation I have every heard. #2, as a retired former Ford Parts and Service Manager, I have experienced Uncontrollable Steering Shimmy with numerous Ford Super Duties. I have been retired for 13 years but remember the Death Wobble issue sounds the same today. Just heard about issues with Death Wobble and my experience in repairs was some braces/brackets were loose/worn where they are attached to frame.
Recommend:
#1- With Truck on ground and not up in air, have an assistant turn steering wheel back and fourth and inspect any movement from any attaching braces and brackets. Any slight movement will contribute to shimmy. Repair as needed. Found some many technicians did inspection with truck on rack. You need stress on components for proper inspection. Most concerning was ranchers, construction and fleet vehicles that put extra stress on Frame components and when customers brought empty, it was to diagnosis customers concerns. Make sure to check Radius Arm and Tracking Arm Braces and Brackets.
#2- Truck raised on rack: Inspect all suspension components
#3- From what I remember and speaking various Ford Engineers, It seems that Frame Braces and Brackets were main cause of Wobbles