Mat and Suzy

I need some new tyres for my 2000 Cherokee. Currently it has Goodyear
Wranglers and I want to keep the same. The front have worn much quicker
than the rears. The fronts have about 1/8" of tread, compared to about 1/4"
on the rears.

So off I go to price some new ones (just two, I figure the rears will last
another year) at Sears, and they tell me they will not fit just two, unless
the non-replaced ones are within a very small tread depth of the new ones.
Can't remember the measurement, but remember thinking it was small. So
small that the non-replaced tyres would probably have to be not many miles
off brand new themselves. The reason they gave was that it might damage the
differentials (although the kid working there didn;t seem too sure that this
was the reason).

So my question is, would it really cause any damage having front and rear
tyres with different tread depths. I would have thought it was so marginal
as to make no difference. Presumably it could only be an issue in 4wd, but
with winter closing in fast here in Minnesota....

And for the bonus prize, if the difference of tread depths matters that
much, am I doing damage driving round with the 1/8" difference I currently
have?!

TIA

Mat