HELP - Vibration / Harmonic at slow trail speeds, 5000 rpm

[mbarryracing]

i was out tonight on my 06rtx with gutted exhaust with my 7 yr old,i let him drive while i was on back and he hit 21 mph and it stayed there for about a mile and my eyes were starting to cross,i never go that slow for more than a second at a time so never bothered me but its normal

This past weekend, I was out riding with the kids and the wife so we didn't get much faster than 20-25 since the trails were rough. It was almost unbearable...
Like I keep saying, i'm glad / relieved in a way that it's not just my machine.