Ford 4R100 Transmission Question/Problem/Advice

The shudder should go away with the fluid replacement. You are doing the right thing by draining the torque converter and dropping the pan to check for debris.
The shudder is usually caused by the torque converter clutch. I have seen this happen with as low as 50,000km on original fluid. Most often fresh fluid and a couple of weeks of driving will clear it up.
I'm not up on the proper fluid to use anymore, but a good Ford dealer should be able to tell you.
Good luck.