According to the Forest River forums, Ford added a new 6 speed transmission to the e450. Also, the drive train angle from the end of the transmission to the differential changes when you stretch the wheelbase of the frame. With this drive train angle change, the coach started a vibration at 60 to 63 mph.
Ford sent a team of engineers to the Forest River assembly plant along with a team of Forest River engineers. It took a little while, but they were able to find the source of the vibration and came up with a fix. Meanwhile, the whole assembly line stopped waiting on the fix to be installed in all the e450's. Two weeks ago, the assembly lines started back up.
How do I know this? My new coach was in the middle of this mess, and will now be delivered a couple of weeks late. I do like having the vibration fixed before I take delivery!