Grade Brake on the Allison as used on Workhorse Chassis,may be left on at all times. All it does is perform a down shift to a lower gear, same as if you did a manual downshift. The transmission senses input and output shaft speeds, and decides if a down shift is needed. You need to lightly apply the service brake to activate the Grade Brake. It will release when the transmission senses it can up shift, or you manually turn off the Grade Brake.
The OD can be left on at all times unless the transmission starts to hunt. I have towed my CR-V thousands of miles with OD on.
The ECM (Engine Control Module) & TCM (Transmission Control Module) talk to each other, you cannot over rev the engine. FYI TCM is on top of the ECM on the motor.
You can tell chassis year, 10th digit of VIN is the year -- applies to all makes of chassis
You should also register at Workhorse.com so you can see any campaigns open or closed on your chassis.