The bed is a little narrow for 4x8' sheets of material on a 90s Ranger. There's only about 41" between the wheel arches and the opening in the bed itself above the arches is about 44" which is not quite enough. (It's wider in the gap above the arches but before the top lip of the bed, but you'll never get anything into that space that can't be disassembled or moved with a shovel...) You could stand your material up on edge, but don't try that with drywall. 48x40" pallets are no problem, though. Or at least one of them at a time. It'll slide right in there provided if and only if you orient it with the skinny dimension between the arches. Ditto with 2x4s if you want to leave the tailgate up. Otherwise your strap game had better be strong because the standard model has a 6' bed and the longer 7' bed is both rare and also not quite long enough. I used to drive a 90s Ranger for work and believe me, I crammed, or tried to cram, every type of thing you can think of into the back of it at some point or another. You can get a lot done with one but sheet goods are their downfall.

The 8' bed work trucks of the era could do what you're describing perfectly, and do so better than today's "full size" trucks while maintaining a smaller footprint on the pavement. That, and you can actually see out of the goddamned windows. I have a 90s Silverado in fleet truck spec, crank windows and a single cab with an 8' bed. I can indeed stack 4x8' drywall or ply neatly between the wheel arches and even close the tailgate on it. Much to the envy of my coworkers, who still pathologically own dinkum 6' bed crew cab so-called trucks that they can't fit anything into the back of, and then destroy the back seats "ThAt tHeY ToTaLlY NeEd bEcAuSe oF ReAsOnS" by jamming them full of tools and dirty materials anyway, so the whole thing is pointless. At that rate you may as well just get a van and call it a day.