Have seen the same problem on a few GM 4X4's. (My 2002 Blazer is currently a sufferer of it, just do not want to tear into it now).
On the ones I have repaired it was due to the front axle interlock not releasing properly. It causes pressure in the drive line that does not allow the T-Case motor to disengage the case. Then the coupling finally slips enough that the splines uncouple. The T-case makes a bang as the torque is released. On most of them the actuator seemed to be the initial cause of the problem. It was not engaging properly and damaged the end of the splines so the collar could not slide freely.
On my Blazer you can see and feel it when it releases, Feels like you hit a low speed bump and whacked a garbage can at the same time.
Saturday, March 4th, 2017 AT 8:31 AM