I swapped in a 4.5" rear into a '91 D-150 and used the 5.5" axles. The C-clip is different in inner diameter and thickness, 5.5" is larger in both. Since the 5.5" C-clips are thicker, I ground them down on the side of a bench grinder wheel till they were the same thickness as the 4.5" C-clips. Of course, with no tone ring in the earlier housing, no more ABS.

I don't see how you could go from 5.5" to 4.5" without making new C-clips.

Just swap in a complete early rear.