said by Bob4:
OK, but it's a serious question: At what point does the difference in tread depth become a problem?
not at all when they are both bald as hell.
basic thing, if the conditions are beyond what the tire can support at the tread level it is at you will have issues, if not, the difference doesn't really matter too much, IE: don't try to out drive the tires.
easiest is just don't drive to the limit of the best tire. if you know the rear ones are bad figure out where their limit is, and drive accordingly.
but try telling your average all season awd SUV driver to drive according to the conditions, you get some amusing replies.
:edit: yea completely forgot the lawyer thing, that's going to be an issue no matter how big/small, front/back the difference is.