If the speedo isn't working properly, the first thing to check is the output speed sensor wires on the rear right side (near the bar code) of the tranny case. Follow the wires and make sure it hasn't been cut, pinched or shorted.
If all is well there, take the output speed sensor off. Watch out cause you'll get wet with tranny fluid (Dexron 6). (Be ready to plug it up.) If there are no metal chips or debris on the magnet inside, check it with an ohmeter on the two pins to see what the reading is. It should be around 6 to 10 ohms. If it's not steady, get a new one. Make sure the pins are clean on the connectors.
Apply some white or build grease around the shaft and "O" ring and re-install. Don't overtighten. The bolt is a "tap-tite" and should need only about 5 or 6 ft/lbs torque. (Run it in and give the bolt a quarter turn.)