Dame is right...
The 4T60-E is mostly
controlled by the PCM. However, vacuum controls the firmness of the shifts (I'm not sure if "line pressure" is the correct terminology, but we'll go with it). If you completely disconnect your vacuum lines the transmission will shift really hard, which is the same thing that happens if you have a vacuum leak or have the vacuum modulator adjusted to allow only slight vacuum pressure (like mine
). If your vac lines are compromised, your shifting line pressure will go up, causing more "jerky" shifts.
This is the reason why aftermarket parts companies advertise "higher shifting line pressure" adjustments to their PCMs, but the effect isn't felt in '97 L36 Bonnies (L67 Bonnies from '97 had the 4T65-E). The only way to change shift firmness in a 4T60-E is to reduce vacuum or install a shift kit. Line pressure is totally independent of the PCM, unlike a 4T65-E, which is fully