The vacuum modulator is located on the front of the transmission. It is visible from the top, toward the left (driver's) side of the vehicle. There should be a rubber vacuum line connected to it, and the other end of the line should be connected to a vacuum port on the intake plenum. You can remove teh vacuum line and check for the presence of vacuum with the engine idling.
If there is fluid leakage into the vacuum hose, the modulator has failed. Even if there is no fluid present in the vacuum hose, the modulator could have problems. They are a common failure point on the 4T60 and 4T60E. Replacing one is relatively easy, even if access is limited. A new aftermarket replacement can usually be found for about $25, but diagnose the problem before replacing anything.
EDIT: - That "4L60/4L60E" should have been "4T60/4T60E" - I got my longitudal and transaxle designations all confuzled in the wee hours. (Nice catch, Max!)