Are you certain the boxes are even different?
I would imagine your '84 has enough miles and time on the clock that there are various worn components that are causing "sloppy" steering, box MAYBE being one of them. (rag joint, pot joint, tilt mechanism, drag link, etc)
If you are certain its the box, I'd check to see if an '88 up box is even different. I'd bet its still the saginaw 800 series, just like our trucks have. Look at their setup though, perhaps they have changed the pot/rag joints. Besides, with IFS, won't the drag link be eliminated, and instead have tie rods? (thinking that cars seem to have tighter steering, even with everything but the tie-rod setup being the same)