just restored a 1972 c-10. converted it from a manual to a 3 speed automatic. tires are new and balanced driveshaft was cut and balanced to fit. new carrier bearing installed. measured distance from side to side to center bearing. aligned all three yokes on drive shaft. front end has also been aligned. but the truck vibrates between 45 and 55 mph. it does not vibrate at speeds lower than that. anything above 55 and it calms down a little bit. any ideas? any info would be appreciated.

 

I have seen this before many times and I will say its tricky but simple. 3 things to check 1 make sure all C-clips for the U-joints are there, 2 I have seen the yolk that houses the U-joints spread from heating them to remove old U-joints and walloping them with a hammer while thier too hot which can cause #3 check for any play in the U-joint by pushing the drive shaft twards the needle bearing cups and not by twisting the drive shaft. it only takes about .020 of an inch to cause a very annoying vibration. Good luck

 

might seem weird but what kind of tires do you have? a friend of mine has 20s on hissilverado but the tires he has on it makes it shake only between 45 and 55...i think the tires are called coopers.