speedo drive / driven gears

[Svanky10]

replaced the instrument cluster and the speed sensor.... fixed the tachometer which read 9 grand...... so thankfully i took care of that problem. hooked a drill up to the speed sensor and turned the a/c on..... speedometer started working! obviously the problem persists in the transmission. all four gears work, with the exception of the overdrive. my thoughts? the drive / driven gears? im not exactly sure how these work, but i have checked the specs on other 700R4 tranny's with my rear end and size tire and the specifications call for something other than what is in the car. the speedo worked for about 300 miles and then eventually quit working. is it possible that these two gears have been stripped or replaced with the wrong parts? anyone with some helpful insight would be appreciated!!