Q: SUV, GMC Envoy, 2004, vehicle has no get up and go! Not shifting into gear! Already changed transmission fluid and spark plugs!

If engine speed increases steadily in response to accelerator inputs, then this is probably a transmission related fault. To give you a precise slate of possibilities though I would need to know exactly which gears (which "shift points") are "missing" and/or experiencing delayed shifting. Generally, there are electrical causes of what you describe plus lots of mechanical possibilities. These range from a vehicle speed sensor in the transmission, line pressure sensor, valve body or shift solenoid fault, hydraulic circuit/clutch pack fault and so forth. Sometimes a faulty throttle position sensor will cause these symptoms. There can also be multiple causes of course. An advanced automotive scanner is typically used to read transmission operating data and that aids in the diagnostic. If you want such a diagnostic, please request service using YourMechanic’s loss of power inspection/diagnostic service request page and the responding certified mechanic will get this resolved for you, right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.