What is making my truck vibrate

by Ron Racz
(Montville, ohio 44064)

I have a 2016 peterbilt 388 with a c15 in it the truck is a glider and it seems to have a vibration or a shimmy when you start down on fuel pedal. I've had a full truck alignment after I had hauled some loads. it is a straight truck.

Could it be a harmonic balancer or what? the mirrors shake and the tire wear is normal, any help ...thanks Ron