2003 Excursion 7.3 all stock.
Transmission acted weird. shuddering, maybe slipping. Added tube of shudder stop- no change. Week later added bottle Lucas transmission fix- slight improvement. Week later dropped the transmission pan and changed the filter. Did not find excessive debris. Same day flushed the transmission with the procedure on this site where the transmission cooler return line is dumped to a bucket and new fluid is added every time bubbles are seen. Took just under 5 gallons. Used Castrol MerconV. Ran perfect, no issue, for about 2 days. Now for the last 2 months, at low throttle it feels good. At full throttle it feels good. But at medium throttle as it shifts into over drive or the gear just before overdrive, there is a minor shudder followed by an odd vibration. What happens if I continue to drive it this way? What else can I try before a torque converter swap? Is it even the torque converter? Is there a valve body or manifold that can be investigated first? please advise how to proceed. thanks

 

A driveline problem may cause what you describe - the driveline is most noticeable when the TC is locked - it locks real close to the 3rd-gear shift in a stock tune.

If you have a scan tool, you can see what the lock signal is doing and what the TC slip is.

The flex plate (flywheel) can cause vibration/shudder issues as well. What happens when you coast downhill at highway speeds, then put it in neutral?