Could be some oil or coolant leaked onto the flywheel or clutch somehow, which eventually got baked/scrubbed away by driving.
If you have a cable-operated clutch, check the cable freeplay at the adjuster. Pull the cable housing towards the front of the truck and screw the star nut adjuster snug against the rubber bushing at the firewall -- or if it's already tight, back it off until it's slack, then make it barely snug again. Press the clutch pedal several times, then pull the housing forward again and back off the star nut adjuster until there's a gap of 15/64" (6mm). Hydraulic clutch is a bit trickier, will get into that if you have this.
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