Is it shifting normally otherwise, which would indicate the rear sensor is good? I don't think I've ever seen one of the 98+ trans speed sensors go bad. The input and output sensors are identical, so you can swap them.
You can use a voltmeter, set to read a/c, to verify that the input speed sensor is working. As I recall, the sensor doesn't read anything in 4th gear but should read in park? It's not a true input shaft speed sensor, but reads one of the ring gears up front.
It doesn't need to be connected to the computer to read with a meter as it puts out a/c pulses, unlike the pre-98 output speed sensor that was just a switch that closed once per rev. I've never tested the input sensor, but I expect you should see at least a few volts at idle increasing in proportion to engine speed. Not sure about the frequency, either. I have tested the output sensor disconnected from the computer before and saw around 1 volt at very slow speed getting up to nearly 30-volt pulses at 60-mpg, pulsing 4x per shaft rev.
