I sometimes get a moderate 'clunk' when disengaging 4WD - and mine is bone-stock. Try slipping the tranny into "N" right before you disengage. This may take some pressure off your drivetrain and allow the system to unlock easier.

Andreas


2001 4Runner SR5 4WD, Dorado Gold, 303,000+ miles
2014 Audi A4, Monsoon Gray
1995 Porsche 993, Polar Silver