As I recall I came across this in Hot Rod magazine probably over 20 years ago.
Anyway this weekend I was working on the El Camino project and got to the drive shaft part. I needed a yolk for a TH400 tranny and after looking around I finally found my last one. Unfortunately at some point it had sat out in the weather and was a bit rusty. The outside cleaned up nice with a wire wheel and some 400 grit.
The inside where the splines are was probably worse, rusty and filled with crud. I wire brushed it and sprayed a bunch of oil and brushed it some more but even with a few light taps with a hammer it would not go further than about 3/4 of an inch.
I recalled the old trick I have used a few times in the past. I went in and got about 1/2 cup of vinegar. I set the yolk straight up and poured the vinegar to the top and let it sit overnight (about 18 hours).
I poured out the vinegar, sprayed it out with carb cleaner and this is the result. All but the deepest rust was gone and down to bare metal. I sprayed some light oil in it and it slipped right into the transmission.
Thought someone might find this useful in the future. ;)