This is from a driveshaft shop.
"In most applications the minimum is 3/4 inch and more than 1 inch is not necessary. This is measured with the rear suspension supported on safety stands or with the rear end on the ground.
Or
With the weight on the rear you should be able to slide the yoke in no more than 3/4" - 1". More than that and it will wobble & damage the output shaft."
I believe they are speaking of how much of the yoke is outside the tranny. The bottom line is how much will the female spline hold and how long is the male spline. Leaving room so as not to bottom out inside or outside.
I agree with T-Nix and I think 46GMC was going to say "go to.... driveshaft shop", but fell asleep :), I also do not think you will find the correct length with the correct ends. I would first be sure it's the driveshaft causing "bad vibes", then check the web for proper engagement dimensions (to verify what I found), then go to a driveshaft shop, tell them your engagement to double check your thinking, then have them use your ends to create proper shaft. In my experience, it's not that expensive.
