Driveshaft problems of the 2002 GMC C2500

I own a 2002 GMC sierra 4 x 4 crew cab duramax diesel pick-up truck. For the last 2 years I've had a condition that felt like a tire was out of balance. This was just after I replaced all tires. This year I replaced all the tires again and the condition still exists. We went through the process of elimination, changing a wheel that was a little out of round, re-balancing and rotating the new tires 4 different times and couldn't eliminate the problem. I then put the vehicle up on a hoist and put the transmission in gear to check for drive line vibrations and found that the rear drive shaft was bent. After taking the truck back to my dealership and confirming the problem with them, a search was run of all the salvage facilities for a used shaft and found that there were none available. Furthermore, every salvage yard that was contacted claimed that this drive shaft, gm part #15109384 was in high demand and as soon as one came in on a wreck, it was immediately sold. This shaft is installed on 2000 to 2007 models. My dealership also admitted that mine was not the first bent drive shaft that has come into their facility. To purchase a new shaft is almost $900. A used one is much less, if you can find one. I worked with gm truck in quality control for 31 years and it is my experience that when a used part can not be purchased through a salvage yard due to high replacement demand, there is usually a cover up going on. Please look into this matter. By the way, my shaft had no visible evidence of damage anyplace. I believe that the shaft may not be able to handle the torque of the engine and will, in time, bend. My truck has 99,000 miles on it now, but the problem started sometime around 60,000.