IIRC, the 4" axle and less than 16" wheels (on an AD truck, at least), is the no-no combination as far as introducing scrub line problems.
I forgot to mention, I'm running 72-~91 Blazer/K10 wheels up front, 15" x 6" because they clear the disc brakes and are 6 lug. I'm not sure what the back spacing is on those.
