I've had the infamous high speed vibration issue ever since I put the TKO-500 in my '94 almost 2 years ago. After 4 different OEM and FRPP driveshafts and multiple rebalances at the local driveshaft shop, I finally decided to contact a shop with a high speed balancer (10k rpm). This brought me to www.dennysdriveshaft.com. I shipped them my latest aluminum shaft and the said it was like a "jump rope" on the balancer. They also stated it looks like it had been made that way. I broke down and told them to go ahead and build me a shaft and make it 5/8" longer so I don't need the spacer. They used my old yokes and total was $305 shipped. Installed it this evening and now it drives smooth as silk. Hope this helps someone else that may be having the same issue.