I'm pretty sure that the tach signal is not from the sensor, but converted within the ECU. On the 7.2 ECU it is, but I don't have any reference handy for a '98/2.5L harness. There was some discussion with the LS swaps between John and VLad using a converter. Some of that is here:
V8 Conversion
If I remember and have time, I'll track it down for you.