Agree with the too low rpm exit. During DFCO all fuel in the manifold is removed. The ECM should begin applying fuel before you need it in order to allow the cylinders to start firing again. Also look for timer value that counts down to DFCO after other conditions are met. They are generally set fairly high.

As a general note, 4L60's don't have decent engine braking in D because the trans is designed to coast when the throttle is released. You need (and are most likely using) a manual range lower than D for braking. Years ago I installed calibrations into 7747 / 4L60E vehicles that would cut fuel over a certain speed as a governor. The engines were stalling if the vehicle coasted down a long hill in D because the TCC the trans overrun clutches disconnect the wheels from the engine. The calibrations needed to be altered so they would pulse some fuel during a long period in fuel cut.