We get this call pretty often: a customer has a drive shaft installed in their vehicle that someone has built or modified and they are experiencing a vibration. This one can be a crap shoot because we don't know if the shaft was built with any sort of tolerance in mind and they usually aren't dynamically balanced.
A customer from Hialeah, FL brought his Dodge Ram 1500 to us who was experiencing said vibration. Upon examining the shaft, it appeared that the person who put the shaft together at least tried to get it straight. However, with it being .025" out of round in the center, we had a pretty good idea of the source of his vibration. He told us he'd called the dealership to try and purchase a new shaft but they weren't offering that model any longer. His driveshaft used 7290 u-joints (also known as the Dodge big u-joint).
After swapping a bad u-joint, we went ahead and mounted the shaft in the balancer. We used a dial indicator to find the high spot in the shaft and straightened it with heat. We were able to straighten the driveshaft to around .008" runout in the middle. From there, we ran the shaft on the balancer and ended up only adding about 20 grams of weight to one side to get it balanced.
Originally, the customer said that the drive shaft made the truck nearly impossible to drive at over 70mph. After quick test drive, he didn't experience any vibration. However, we always like to make sure we've remedied the problem so we asked that he call us to let us know if everything went smoothly. After driving back to Hialeah from Ft. Lauderdale, he called in to say that the truck was driving like new!