I just looked at the 1987 - 1989 service manuals.
ECM = Engine Control Module
CCM = Cruise Control Module
VSSB= Vehicle Speed Sensor Buffer.
87-89 have cable speed heads with a VSS Buffer Amplifier with the optical pickup screwed to the speedo housing for cruise control. 1987 & some 88-89 use it for the ECM as well.
The optical VSSB on the instrument cluster is the same type used for Cruise III cruise control. The Optical VSSB was all discrete thru-hole components. It's the same type of unit from 1983-1990. According to the Fiero guys the buffer in the later units is improved over the early units. Rock Auto has replacements for these. It's a 2 pulse optical unit. There's a light chopper (a reflector behind two blades or something similar) inside the speedometer head. The sensor assembly on the end of the ribbon shines an LED beam on the reflector that gets interrupted by the blades as the speedometer cable turns. The light is reflected back to a phototransistor twice per revolution of the speedometer cable. This signal is then sent to an amplifier and buffer circuit which turns the signal into a "on" and "off" signal the ECM can interpret as vehicle speed. For every revolution of the speedometer cable, there are 2 pulses sent to the ECM. GM speedometer cables are calibrated to turn 1000 revolutions (or 1001 depending on who you ask) per mile. So the input the ECM or cruise module sees, is 2000 pulses per mile. (PPM)
The optical VSSB was all discrete thru-hole components (TO92 package transistors, resistors, capacitors). None of them are potted so they should, in theory, be repairable unless the LED & optical pickup sensor on the pigtail is kaput.
I won't go into troubleshooting one of these. They can be purchased new for around $50.
Wiring to the edge connector from the top in the picture... Power, Ground, narrow gap, ECM Signal, wide gap, CCM signal. On your Oscilloscope the output signals should be 0.556 HZ / Mile square wave.
Some but not all 88 & 89 RV series trucks have a VSS sensor screwed into the transmission or transfer case with a DRAC or VSS buffer (not the same as the optical unit) in the cab. You'll have to look to verify what you have.
90?? & 91 RV series has electronic speedometer with VSS in the transmission or transfer case and a DRAC in the cab.
The 88-91 VSS Amplifier Buffer or DRAC module, if you have one of them, should be mounted near the ECM under the passenger side of the dash.