If no one who knows the solution to a topic, sees it in the first day or two, newer posts push the older a few pages back in the forum post order, and a lot of people don't look at more than the first page or two in the posts list.
Like many things, to troubleshoot the speedo it helps to understand how it works. There is the rear differential speed sensor (I forget which model years it changed on but I think it's still there for '97 & '98 (?), producing a low voltage AC signal when the wheels turn the axle and differential. You can check this with a multimeter. It uses wire and connectors to go to the ABS module. You can also check those. You could also do it the other way around, as the sensor in the diff is several hundred ohms or more, so you could measure for this resistance at the respective pins on the connector to the ABS module, and again at the sensor. I've posted the pinout for the ABS module connector recently in the forum (in the last several weeks) so a search through my posts should find it.
The ABS module translates this low voltage AC signal from the speed sensor, and produces a signal (on "VSS Out" connector pin) to drive the speedometer, which you can also check, then wires and connectors at speedometer, again can be checked, then whether speedo itself is working with a signal getting to it.
There are wiring diagrams in my sig below, pay attention to two named anti-lock-brakes, and instrument-cluster-instrument-cluster-circuit-1-of-2, in particular.
Don't forget the check the fuses, often it is the first thing to check because it's easier. Sometimes they aren't blown but the contacts have corroded or tarnished after this long and it helps to remove, clean off, and reinsert a few times to clean the socket contacts a bit too... or use a spray on, contact cleaner.
You might also be able to pull up a trouble code with a scan tool, though it has to be capable of Ford specific codes, not just OBD2. You might also be able to use a scan tool that shows real time data and monitor whether the rear speed sensor is providing a signal while driving.
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