How does the vacuum modulator effect shifting in Drive/overdrive?

Hello, your 1993 Chevrolet truck does not have a vacuum modulator. The transmission is electronically controlled. If you have an issue with transmission operation, a test of the components of the system is needed. This testing is not as advanced as newer models, as it has pre-1996 diagnostic capabilities. The torque converter, for example, may not be locking. I would advise referring to a local qualified technician or transmission repair shop.