I'm having a shift problem in my 1996 suburban

There are electronic controls involved here, but also hydraulic function. When test equipment is connected to the vehicle, you can monitor things like vehicle speed as recognized by the engine's computer and in turn by the transmission, shift solenoid position as requested by the engine's computer, as well as actual shift solenoid position. Once you can monitor these things, you can find out if the vehicle is calling for the gear change, and just not engaging it, or if it doesn't even call for it for some other reason. I would recommend an experienced technician test your vehicle to determine the cause of your concern.