P38 replacing tyres

Hi, The time has come when i need to buy 2 new tyres for the front of my p38 dse. I have the 20" stormers from a sport with 295/40/20 tyres. However i have been told by some tyre dealers that some 4x4's like the bmw x5, and mitzubushi need all 4 tyres replaced at once as not to cause any damage to the diff.

Anybody know if this is the same on a p38, Range? I have about 3.5-4mm on the back and obviously will have 8mm on the rear when i get these fitted.

Sounds like they're just trying to sell tyres! Surely the whole point of the diff is absorb any difference in rotational speed between sides or front/rear. If the difference was too big it might stress the viscous coupling, but a few mm would be ok, as it's no different to having slightly differing tyre pressures. I'd switch the new ones onto the front as they take the heaviest wear, and then you'll get more life out of the worn ones on the rear.