If it's one of the two transmission sensors, if the speedometer works normally when it's stuck in 2nd gear, then it's the input speed sensor. When the output speed sensor messes up, the speedometer will quit working. Unlike other powertrain sensors on these cars, the aftermarket speed sensors generally work fine.
1" open end wrench works well. A 1" socket with extensions can work, but sometimes there's too much stuff in the way to get a straight shot. The sensor bodies are plastic, so just slightly snug them - their o-rings seal them.