I just had to change the "Transmission Range Sensor" switch or TRS in FordSpeak on my '97 3.0 Taurus(at 163,000 miles).
Anyone care to guess how I knew it needed replacement?
Here's a hint - there were no transmission or driveability problems for the couple of weeks leading up to doing the job(I knew it had to be done, I had the part in the car with me, it just took a while to get around to it).
Rob