If you had 2 tires that were more worn than the other 2, where would you put them? Front or back?

All 4 tires are the same, but the rear 2 are much more worn than the front 2 which are about a year old and have a good amount of tread depth left. The rear 2 are however much older and have less tread depth although I dont think they need to immediately be replaced. My question is should they be in the back or the front? I am starting to notice more and more skidding sounds but im not skidding. You know the tire squeal when you take corners hard. Im not slipping though but I know its not far off. I dont have the money to replace any they are expensive. Oh yea and if it matters I have an all wheel drive ford explorer (97) its not 4-wheel drive, its all-wheel drive. There is a difference. Thanks!