Method for synchronized four wheel drive low range shifts with vehicle in motion

A method of controlling a four wheel drive (4WD) low range shift in a vehicle, the vehicle having a powertrain including a transmission that drives a transmission output shaft to provide drive torque to a driveline including a power transfer unit (PTU) that delivers torque to front drive wheels and a rear differential unit (RDU) that delivers torque to rear drive wheels, the PTU and RDU selectively rotatably coupled to each other through a propeller (prop) shaft, the method comprising:

• commanding the PTU and the RDU into a neutral position based on a determination that a 4WD low range shift request has been made;
• commanding a clutch at the RDU to selectively engage to a degree that communicates a desired amount of torque from the prop shaft to the RDU;
• determining that an output speed of the transmission output shaft is synchronized to match a speed of the RDU;
• determining a first speed of the prop shaft;
• commanding the RDU to shift into 4WD low based on a determination that the prop shaft first speed satisfies a desired first speed;
• commanding the clutch at the RDU to open;
• determining the output speed of the transmission output shaft is synchronized to match a speed of the PTU;
• determining a second speed of the prop shaft;
• and
• commanding the PTU to shift into 4WD low based on a determination that the prop shaft second speed satisfies a desired second speed.