Cory, it occurred to me that a single chip from your broken speedo shaft or something it whacked into inside might have lodged in a synchro sleeve. If so, no matter what manipulations you go thru with the internal/external linkage, such a chip- if it exists- will not be dislodged without disassembly. I hope this guess is wrong.... But I have seen such things happen before. There's just not much spare room inside those boxes!
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