Well I thought that the sprotage 4WD system uses a EMC (Electric Magnetic Clutch), where the Transmission Control Module (TCM) distributes the required torque between front and rear wheels. The degree of drive to the rear wheels is governed by a number of inputs including the amount of torque commanded by the driver, individual wheel speed/spin, steering angle. In fact, there is always some drive being transmitted to the rear wheels. Pressing the button simply engagues the clutches, however, after approx 30km/h, the clutchs are ramped down, and the car goes back to being just as it was before the button was pressed. As you slow again though, the clutches will re-engage bring back on the 4WD.
The transfer case is always sending the power to the rear wheels. It is the clutch coupling system at the rear diff that controls the amount sent to the rear wheels.
I would use the 4WD lock button in slippery conditions (i.e. snow/ice,mud, dirt tracks, even heavy rain and smooth roads), but if you are over 30km/h, then the 4WD system is essentially off.
This system is not like a traditional 4WD system that required you to be stationary to engage, particularly if you were transferring from hi 4 to lo 4.
I have had my car on a 4 wheel dyno, and as soon as the fron wheels started to turn the drums, the rear ones did as well, so this to me indicates that there is always a level of torque to the rear wheels, else they would have not turned at all.
The clunk you felt is the disengaging of the clutch system. Wait to you do spin and feel it engage when you don't expect it.
