I beg to differ, I remember hearing my rear u-joint popping before I put on the ext. slip yoke, that's when I was only at 3" of lift. the extended length is in the yoke not the female shaft. This allows more movement in the u-joint.
Now I have +6", ext slipyoke, and tcase drop, and ext rear shacles. I have vibes between 45-55 MPH then they go away, and the vibes are only noticable in the rear view mirror.
Most likely you will not be able to eliminate all your vibes. Just plan for and SYE, I have mine, but need to get off my lazy A$$ and put it in.