At your mileage both the camshaft and the lifters are probably well worn and need replacing. The noise likely comes from lifter/cam lobe wear that has pushed the preload out of spec which has led to the noise.
You'll need to replace BOTH the cam and the lifters since throwing new lifters on a used cam will shorten lead to accelerated wear of both components. However it's a pretty involved job to get to the cam with the block still in the truck so you may be better off switching to a heavier weight oil to help with the noise. It doesn't make sense to throw so much money and time at an engine that's considered very high mileage.
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