It does sound like a dry bushing but you have covered all the usual suspects. The only thing I can think of would be trying a differn't speedo adapter then see if someone can loan you a differn't drive shaft. You cant feel it through the shifter? Does it stop when you kick it into neutral?
Gus
64 Plymouth Savoy 493 Indy EZ's by Nick at Compu-Flow 5-Speed Richmond faceplate Liberty box Dana 60