63 dart question

If Dan's suggestion doesn't cure it, check the cable itself. They are not known for going out of adjustment, but something else may have happened or the cable may be
dragging for some reason. Reverse is pushing the the cable into the trans to it's limit of extension in operation, not likely but a small chance it's binding for some reason.
Is the trans engaging reverse at all? Good luck, hope it is just a "gummy" mechanism..