Stuck on this same problem (for over a year now).
Video Here.
Here's the scenario: 285k miles. New motor at 198K (10 years ago). Trans was never an issue.
I can manually shift it down to 2nd, and it does, no shudder. In Drive, any kind of incline or pedal and it feels like it's coming apart.
New Kickdown cable. Also, I've adjusted the kickdown per FSM, tried all the way to both ends and everywhere in between. Seems to change the shudder a little.. but never goes away.
Replaced Solenoids.
New Filters, New gaskets. New Fluid. Flushed, new fluid again.
Replaced Valve body.
Fixed some charred wiring when I replaced the motor. Been wrapped with heat wrap since, all still looks good from the outside. no probems in almost 100k miles.
Manually shifting it does fine. Shifts great.
U-joints look good. Doesn't really do it, except when starting off under a load. All other gears seem good to go.
I'm blaming any hair loss I have on this truck.. don't get me started on the AC system in my 60. Any ideas would be very much appreciated.