Considering your current RPM's at 60, I'd say if you want to maintain any kind of highway speed you need to stay where you're at now. I would not pay to go from 4.56 to 4.88. I think your only real option is 5.13's, and that means either highway at 55mph turning high RPM's, or a tranny swap as well.

Hey BigNorm, hope all is well with ya!
He did post "one tons" on the axle swap, so I would assume D60 or better in front.