Depends how fancy you want to get. You can get custom Deaver-type springs for a great ride and lots of $$, or go more basic (like I did) with just an updated spring pack. I got mine from ATS springs, with a 4/1 setup (4 regular leaves and one overload) upgrade from my sagged, stock 3/1 setup. Rides great and really got my van's rear end back up where it should be. I used to drag my bike rack on bigger dips (driveways, etc.), but haven't once since the upgrade. Money very well spent and the van rides much better now. As I posted in your other thread, it was about $600 installed for the new springs, shackles, bolts, etc.
I'll probably update the front springs one of these days, but I don't think they are that bad. I upgraded to Bilsteins all-around several years back when I upgraded to the 2012 front suspension.
See my review on FTE vans section, with before/after pics: https://www.ford-trucks.com/forums/1...on-review.html
My van, loaded for traveling, is 8650 pounds +/- total, 3350 front, 5250 rear, as actually weighed about a month ago.