Backspacing should have nothing to do with the caps not fitting. The wheel mounts in the same place no matter what. I believe it is the axle hub that sticks through the wheel are longer on the older trucks compared to the newer trucks.
I have a buddy with a dodge that is having the same issue and its because the axle housing sticks through further.
That's because the axle width changed.
I have a pair of spacers I don't need. Pm me if you could use them