What gear set should I go to- 4.56 or 4.88

[maxacceleration]

Good move REF. It only makes sense... go up in tire size and lower your gear ratio to keep close to the same factory ratio/rpm's @ 60 mph. If you tow big weight you can slightly over lower to pick up 'pep'. If the weight is usually light, diesel torque will compensate and you can under lower.
You are not missing a V-10. I call mine a 'circle track' motor as it depends on lots of rpm's to make its torque & hp. 420 ft lbs of torque 'peak' is at 3250 which the motor needs to spin at. And more is needed with 4000-5000 rpm needed to ascend passes with authority. Enjoy your 7-10 mpg with it (mine van weighs 8700 lbs).
A diesel will pull more weight easier, every time. Get your diesel at its best 'tune'.
I went from 4.10 to 4.56 with 295/70-17 tires running about 2200 rpm at 70. I could have gone to 4.88 as the V-10 needs rpm's anyway.

Proper rear gear is the single most important thing for a 8000 plus pound Ford van.
IMO, over lower on gear ratio and cruise at 65 mph.