Chevy dropping out of overdrive

Check engine light is it flashing or On steady-if so you need to it have scanned for OBD2 code/s and comeback with it.

When was the last fluid/filter change. Probable areas that can cause your problem if the computer don't get the right signal from these sensors: coolant/throttle position/vehicle speed/Manifold absolute pressure/brake signal/ TCC solenoid. That would cause it not to shift correctly thru shifting solenoids. Best deal is to have the tranny pressure tested-this will identify problems internally.