Huzzah!

With creative use of three different screwdrivers, I got the plug off.

After some troubleshooting, I discovered that the shifter has to be in Drive before I can control the governor pressure solenoid. Otherwise the sensor always reads 0 psi.

Once I moved the shifter however, it appears that I have full control over the solenoid.

The highest voltage I got from the gov. pressure sensor was only ~2.6v (~70psi), and that was with either zero current commanded, or with the power supply line to the solenoid disconnected.