2011gmc sierra shifting problems

First thing is to check the color and level of the transmission fluid, if black it's too late and will need overhauled. If it's still bright red and full cold engine idling, then most likely you have electronic problems. Could be shift solenoids, TCM, low internal transmission pressures due to plugged filters.

Sounds like you've lost torque converter clutch lock in and overdrive. A typical GM failure. It can be repaired but as Gene said if the transmission fluid is dark or black it's best to replace the transmission assembly. Jim