Rear tyre wear

I have a 53 reg TD4 with 75k which needs a new pair of rear tyres as they're down to the indicators, but as well as this they have both developed an unusual wear pattern on the outside edge of the tyre. The leading edge of each tread block is higher than the trailing edge, so producing a 'saw tooth' effect.

I have three questions:

1/Can anyone give me a simple explanation as to how this happens?
2/Is there any way of preventing it?
3/Is it an indicator of something more serious mechanically?

I've read various pieces of advice such as you should always fit four of the same type of tyre, or that new tyres should always be fitted to the rears and part worns moved to the fronts. I've also read that Land Rover instructed their mechanics to swap rear tyres to the opposite sides of the car if they encountered the problem, surely this would only hide the problem for a few thousand miles and wouldn't be advisable for directional tyres anyway.

I've done the VCU tippex test recently on a wet field and both marks were well out of alignment after it so the VCU doesn't appear to be seized. Apart from the occasional knock/crack from underneath there doesn't seem to be any unusual noises from the rear diff or wheels.

The car is fitted with Goodyear Wrangler HP 215/65R16 on both rears and Bridgestone Dueller H/P Sport 215/65R16 on both fronts.

The outside edge wear you describe sounds very much like 'feathering' a fairly common phenomenon caused due to the tyre being unable to roll in a straight line because of other factors. The tyre tends to 'scrub' along the road causing the surface of the tread blocks to overheat and stretch in the 'sawtooth' pattern you describe. Before investing in new tyres I suggest having a full 'alignment' check carried out and with luck the apparent excessive 'toe-in' of the rear wheels may be corrected by adjustment.

The explanation given to me by Landrover is that the Freelander, because of the IRD ratio, drags it's rear wheels slightly. Of course the VCU slips, but this still results in some drag being experienced by the rear wheels. I had the same sawtooth effect. Since then I removed the propshaft and am driving in mondo mode and no longer have this problem (2 sets of tyres later). My last set was 195/80R15 Wrangler tyres and they wore down perfectly to the indicators. Wheel alignment can also play a part as decribed by Dann above.

The IRD ratio problem was corrected in or around 2001 I believe but as mentioned a full 4 wheel track check is needed, also check handbrake cables are not siezed partially, causing rear brakes to bind, even slight binding will cause "tread blocking" as some call it.

If you have tyres with a block tread pattern this does happen. Ask anyone with a series landrover. As the bloocks hit the road they slowly distort. You have to swap the tryes left to right every few thousand miles.
If you have more 'road' type tyres it shouldn't really happen.

Thanks for the suggestions, I think I will try a new pair of rear tyres with a 'less blocky' tread pattern to start, if unusual wear continues I'll get the alignment checked and if it's still the same after that I'll just have to accept it as a characteristic of the car.