Both sensors are on the gearbox, I'm not 100% sure if the auto gearbox car has 2 sensors and the manual 1.
Sensor locations, (the files take a few seconds to open).
http://www.turbosupras.com/pdfs/JZA80%20TSRM/DI.pdf#page=413
Also
http://www.turbosupras.com/pdfs/JZA80%20TSRM/DI.pdf#page=249
I would be looking for a bad connection at the sensors, multiplug connectors at the speedo and the ecu. Unplugging them and making sure they plug in okay and make a good connection.
With the pdf files above, at the top you can search for specific words like "speed sensor" , click find next and it will take you right through the manual where it's found the words you searched for.
Do a search in the diagnostics section in the main manual below, I've just started searching like this, it's a very useful function as the workshop manual is absolutely huge.
http://www.turbosupras.com/pages/pages/technical/jza80/a80tsrm.htm