Any spring shop can add a helper leaf to raise the rear. Adding an all new progressive leaf pack will not only raise the rear, but it will also improve the ride. Depending on how much the rear is raised, you might need slightly longer shocks too.
Weldtec is another source to check out for full leaf packs and add a leaf's. Different van but I've been really happy with their progressive leaf pack for my Chevy.
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2015 Chevy Express 3500 Duramax
w/ Quigley 4x4 & Agile Fox shocks
Sold 2005 E350 Chateau
Quigley with Agile RIP, 6.0 PSD