Service Manager said they think it is a "stretched differential" issue. Has something to do with longer rigs.
Checked a couple other forums. Some orders for longer motorhomes on 2016 E450 chasis were put on hold because of the "vibration". The 2016 has a 6 speed transmission, but the " manufacturer" wasn't notified of the difference and used 2015 specs. How accurate this is I don't know, but it sounds like the orders have been released and producrtion is starting up again.
Doesn't explain why my 2016 on a 2015 chasis has a drive train issue though. Hopefully, I will find out more when I pick it up tomorrow.
Dave and Chris
US Navy (Ret),