So I've looked into it a tad. When I first bought the truck neither worked, and it never really bothered me. But I figured hey, may as well fix it (I kinda wanna see my miles lol).

But anyways what should I be checking? Fuse #18 and the sensor on the rear. But should I check out anything else?

 

After looking at the 1995 schematics the rear sensor does input to the PSOM. Check fuse 8 inside also.

The 20A fuse directly under the PCM Diode in the fuse panel under the hood powers inside fuse 18 so check it too.

How The Circuit Works
The Programmable Speedometer/Odometer Module (PSOM) receives a speed signal input from the Differential Speed Sensor (DSS) and uses a programmed conversion constant to convert the signal to the standard 8000 pulses-per-mile speed signal output. In the vehicle, the PSOM and Rear Anti-lock Brake (RABS II) Module are parallel users of the DSS signal. The speed signal output is proportional to the road speed of the vehicle. The Programmable Speedometer/Odometer Module supplies this signal to the Speed Control Amplifier, the Instrument Cluster (Digital), and the Powertrain Control Module (PCM), all of which require vehicle speed information.