Sounds like the plastic driven gear (that goes in the side of the tail housing) is not engaging the internal drive gear. Remove the side driven gear, examine it, take a flashlight and look inside the hole to check the location of the drive gear. If every thing looks OK, put the side gear back in and turn the tranny tail shaft (the driveshaft) and see if the shaft of the speedo gear also turns. If the speedo gear shaft does not turn then the gear teeth may not be engaging the internal drive gear.
Let us know what you find.
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