Need to get a 2 piece...mine did the same thing. It starts to spin above the "safe" rpm and starts to flex. I had a 2 piece put on and instantly fixed no more vibration

 

Well I finally just got my drive shaft in and what I nightmare. To correct all the vibrations after the new shaft was in was hell. The trans needed to lift up about 1.5 inches, a lot of spacers were needed and the pinion ended up getting shimmed to point up. But all is good now

 

07' and AWD, stock truck and I get a vibration in the rear at the same speed. I thought a tire was out of balance for a while, had that checked and still the same problem.