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If it is a manual, put it in 4th, and rock the truck. That will move the flywheel a smidge. Then maybe the starter gear teeth will engage.
Sounds like your starter or ring gear teeth are worn, or the battery is low on charge, or the starter relay semi-functional. All the above can cause the starter to clank, and fail to turn the crankshaft. The solenoid on the starter, if separate, could also be failing. The older starters had a 'ramped or tapered bump' on the starter which functioned the same as an external solenoid.
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