It seems clear to me now that if you change the u-joint center to center
distance, the driveshaft angle also changes, and this forces the U-joint to
work outside its intended arc. In addition, I believe it causes an out of
phase condition...a driveshaft speeds up and slows down twice each revolution,
as the u-joints flex in an arc with a center of rotation that's different
than the center of shaft rotation..it's very complex to grasp this, but it's
true. (it's the reason Cardan joints are used in modern 4 X4's.)
I now believe that the overdrive unit made the shaft angle so acute that the
shaft was wrapping up. There's a URL on the web which I don't have at my
fingertips that explains this with neat graphics. I'll post it later so that
this stop-start action is more understandable.
OTOH, if the overdrive in question was the one which just "plugged in" to the
transfer case with no shaft modifications, like the Fairey unit for the Land
Rovers, then, as Emily Litela used to say, "nevermind"!
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