No argument there. Sometimes it's hard to know exactly what you've got in the bed, weight wise. When we had our 1/2-ton truck I probably overdid it a tad a couple times when hauling dirt. Think my estimates were close enough I wasn't over much, though.
20 sheets x ~75 lbs = 1500 lbs, I weigh 145, puts me at 1645 lbs, about 100 lbs under GVWR. So strictly by the vehicle's ratings, it should be fine. That said, if I had a project big enough for 20 sheets of plywood I'd probably need a lot of other crap too and I'd just rent the Home Despot truck for $20.
Does your van's curb weight include fuel ?
1500++ pounds is actually plenty for a half ton pickup.
I tend to be a little more careful these days due to my vast experience with overloading vehicles
(ever blown a brand new tire in the middle of the Yukon Territory...?)
We have to be damn careful with weight in our little FWD car. 50# feed sacks add up fast.
I just never got into the mini-van thing, but I'm sure they are perfect for lots of folks. Neither do I like full size vans very much.
And coincidentally, yesterday I cleaned out almost 20 years of stuff from the cab of my 1976 GMC, and we drove it last night just to warm it up after a couple years of just sitting most of the time. It has the most comfortable seat of anything we own.