The E tire has a 10 ply rated carcass (sidewalls) with 80 psi pressures to help stiffen the sidewalls.
The D tire has only 8 ply carcass and 65 psi which leaves the tire with a flexy sidewall (less stiff).
Granted some D's may have more load capacity but the E is the tougher/stronger/stiffer tire. Both have a two ply carcass but the D tires have a thinner mil thickness material.
If the truck is used to hot rod around making smoke and burning rubber like kids do and only carrying a small load then the D will work fine.