Never heard that or had that problem and in summer Canberra is a hell of a lot hotter than the Whitsundays (a lot colder in winter as well).
The IRD is just a combined transfer case/diff so outside temps are not an issue.
PS - the IRD cannot lock unless it is broken - I assume you are talking about the Viscous Unit (VCU) - again I have not heard of heat affecting it - the fluid thickens as a result of shear forces placed on it - not heat like the Viscous Coupling in a cooling fan where the fluid thickens as it gets hot.
Garry
				
			 
			
		 
			
				
			
			
				REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101 
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
			
			
		 
	
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