Heavy vibrations

[alsriv2]

I had a similar issue with an '83 C-10 I used to own. Seems when I replaced the Ujoints, doing the exact same method I had used many times, I bent the driveshaft. Not enough to visually see damage, but enough to cause a vibration at a certain speed on the highway. Ended up having to replace the drive shaft...no other way to solve the issue.
Worst part? I didn't need to change the Ujoints...I did it only because I was heading out for a drive cross country (Northern Maine to Arizona). Everything was fine before I did that work.