I believe that the Dual Path is operated by linkage. With that being said if the trans. has worked previously my 1st. guess would be that the linkage is somehow bound-up/rusty. You could pull the trans. & have it rebuilt only to have the same problem afterwards.
Just a thought. Before I will usually get TOO involved with something I try & diagnose the heck out of it using the KISS principal, Keep It Simple Stupid.
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