Bad Vibes

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I have a friend who's working on a 72 long bed. He took out the two piece drive shaft because he preferred a solid one. He got one from a salvage yard and had it cut to fit his truck. Problem is when he goes over 40 mph it vibrates wildly. Another shop verified that the work was shoddy on the first drive shaft. He had recently bought a late 70's Buick for parts and as luck would have it, the drive shaft in it fit perfectly, but it vibrates at even lower speeds (around 25). He said that he had driven the parts car home and there was no problem with the shaft. Any ideas on this one? He did have 20" rims on all fours but switched back to the originals.