OK the picture with a read arrow is not very clear..
www.avigex.ca/xport/modulator.jpg
picture from '93 3.8 with same 4T60E transmission
..it is located on front of transmission torque converter bellhousing , clearly and readily visible on mine. You might have to remove some intake air ducts to see more clealy. There is a vacuum hose going to it...With engine running in park, vacuum is maximum and disconnecting the vacuum tube drops vacuum and is like going full throttle from a transmission point of view and this increases hydraulic pressure in tarnsmision which normally sounds as a slight increase of transmission whine...but that can be covered by ambient noise.
BTW, yesterday I tested the minimum speed at which 4th gear engages on mine, and it is around 77kmh. Tcc had applied in 3rd gear around 65kmh.