I have this 'thing' where every so often I put the terrain response into Rock Crawl (but not drive anywhere). My thinking is that it checks low ratio can be engaged and the diffs lock, i.e., no dormant failures.

Am I being paranoid or sensible? My wife thinks I'm more likely to wear it out and cause a problem!

no, I do the same. I also engage low range on my Defender every so often.

it's good to have the systems work themselves every now and then  --
2010 Defender Puma 90 + BAS remap + Alive IC + Slickshift + Ashcroft ATB rear
2015 Range Rover Sport Supercharged V8 HSE Dynamic



Defender Puma Workshop Manual: https://bit.ly/2zZ1en9
Discovery 4 Workshop Manual: https://bit.ly/2zXrtKO
Range Rover/Sport L320/L322/L494 Workshop Manual: https://bit.ly/2zc58JQ

Engage low range and drive it forwards and back about 30 yards. Sticking it in rock crawl will usually lock the centre diff too. That's everything moved.

Engaging low range and driving it back and fore is part of the service - but I bet it never gets done.

Of course, the best thing is to take the vehicle off road now and then and give it a proper bit of use! No point worrying about whether things work if you never use them anyway.  2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

I also go through the full range every so often just to excercise everything. 2019 5.0 SC SVA (Current)
2012 5.0 SC AB. Sadly written off by a campervan
23 plate Focus ST (Current)
1942 VEP Ford GPW Jeep (Willys) (Current)
1943 Whites M16 Halftrack (Current)
16 plate Eurofighter Typhoon, 2 x Eurojet engines with 20,000lbs thrust each. 1380mph