If you only have 1" of free travel from fully collapsed on the driveline with it in the mounted position and sitting normally, you could be bottoming out the driveshaft with suspension flex while running down the road. Maybe you could spray some paint on the splines on the male end of the slip yoke as it is sitting still, and then drive it and see how much paint is stripped off from the suspension flexing and shortening your drive shaft to see if your drive shaft is bottoming out in certain instances.
i feel that might be the case too. and i agree with yout...but i had a talk with my local drive line shop and they said it wouldnt really matter... it shouldnt be vibing at those speeds. if it was the case, then it would vibe at everytime i acceleration....
