Stacking drywall sheets in the bed of a smaller pickup
Most small trucks have indentations or molded rails at the same level as the wheel arches which accept crosswise two-by lumber for just this purpose.

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If yours doesn't, three simple platforms comprising said crosswise two-by and some legs should do fine.
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I owned a classic 'Yota during some of the years I was a home builder. This worked well. You can't put 40 sheets in a small truck anyway, so the platforms don't need to be very robust. I even carried a modern snowmobile on mine.