@mgmetalworks : thank you so much for such a clear and fruitful explanation.
So we have two options : go with a kit like Timberline offers or a complete disassemble of your transmission.
FWIW I had my 4R100 converted to 4WD by a local transmission shop because I knew it was a good runner and it only cost me $400 parts and labor, I pulled the trans myself and dropped it off.