RX4 tyre query

[paul in normandy]

RX4 went in for CT the other day (french MOT) and was failed on the 2 rear tyres. Ordered 2 more, and when I went to the garage the mechanic (a proper mechanic not some unbarbered youth that fits tyres) said that it was advisable to replace all 4 tyres at the same time, to avoid potential problems with the gearbox / transmission.
I had never heard of this, further discussion with the mechanic and the person at the CT station revealed that this advice applied to all vehicles with constant 4 wheel drive, but not to those with selectable 2 / 4 wd.
What's the concensus of opinion on here about this?

[myglaren]

It is something that I have heard many times in the past, particularly with regard to Volvos and Range Rovers.

[Sloppysod]

It makes sense, well almost, I can see their point when 4 wheel drive cars had 'fixed' transmissions, gears & propshafts, and it was an option to have a limited slip differential, but is it still relevant these days, where most have constant vilosity (fluid) couplings somewhere along the driveline? Just a thought.