I agree with the 15 X 7 - 4.25" backspacing wheel. The mid to late 80's Chrysler 5th Avenues all used that wheel. You can tell them by the extra holes between the bolt pattern that was used to hold a center mount to hold the wire wheel hup caps. I've bought them as cheap as 5 bucks in bone yards.
zandfull, you also mentioned your fronts are 15 x 4's Did you have one of the wheel companys like Wheel Vintiques, Rally America, etc. build them?
