I had that problem on my wheel drive shafts. Turned out I was using the drive shaft yolks from a Maxx 3.3. They are a little different from the Revo yolks. This post from an old thread of mine should explain better what I'm talking about.
The X model is what you want I thought. It is the "universal" yolk allowing the shaft to be installed on either the Maxx or Revo diffs. If you have the one shown at the bottom in the picture then this is probably not your problem.
I can't decide if its more fun
to make it...
or break it...