GMC Troubleshooting Questions? Ask a Mechanic for Answers
Your transmission is shifted with a combination of electrical signal from the computer and a hydraulic valve moving in the valve body. Since you just flushed the fluid it is possible it dislodged a small amount of debris that is making the shift valve stick. But it could also be an improper signal from the computer. Before any work is done it is important to have it hooked to a scanner to verify whether the computer is commanding correctly, if it is then it is likely a sticking valve, easy fix. If it is not commanding correctly then electrical diagnosis has to be done to find out why. Any trans shop should be able to check it out for you.