If the vibration was coming from the front end, you'd have to be in 4wd, have a bent wheel, or loose wheel bearings to cause that.
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.
