Some words from Haynes manual.
"has an important effect on shift quality. If your vehicle exhibits shifting problems such as slipping or harsh shifts that are either too soft or too harsh check the vacuum supply to the modulator". Says vacuum should measure 13-17 at the modulator idling hot in drive brakes applied.
The pictures in my book are zoomed in very close so not obvious but studying my car and features in the picture it appears to be on the front of the transmission looking down between the drivers side dog bone and the air cleaner.
It looks like one of the easiest things to change on this car. Disconnect the vacuum line and then there's an obvious mounting bolt and nut and you use a magnet tool to remove the valve. You need a new oring for installation.