I would get a good vaccum guage and test the vacuum of the actuators behind the wheel, pump it up and check it to make sure they are both holding a vacuum and not leaking down. If one or both are leaking down at a considerable rate then you do have a actuator that is not allowing proper vaccum to build quickly when you turn it off, the grinding you hear is the acutuator partially unlocking from the axle, it stops when the vacuum has built up enough to over come the leak and completely dis-engage. If that is your problem, replace the problem acutuator. But if you have replaced lines and solenoid, the only thing left is to check the check valves and check the actuators.