2000 Ford Excursion V10 Vibrations at speed

Hello,

   Recently picked up a 2000 Ford Excursion V10. Tires are 35" MTs. 6" lift. I have yet to really dig into why it begins to vibrate at speed. Im hoping w this info on the widening of the speed range at which this occurs, it may shed light on the possible culprit(s). I've read where it could be anything. Have followed a thread where a guy went thru everything, bushings, tires, bearings, driveline components and was at his wits end because it still was there. I'm hoping w my update here my issue is easier to rectify. When I got it, done as is, the vibrations occurred from 59-70mph. It's not in the steering wheel fyi. More so feels like the body, console , seats. I guess  all around me but not the steeringFord Excursion wheel. I'm on a 400 mile round trip. As of now, it starts at 55 and thru 78 and was still present. I didn't take it any higher. Now this harmonic sound / vibration I can hear in the tires. Pretty sure this either didn't exist or just not noticeable before. I made a video which this can be heard. I'm really wanting to lean on a tire balance would fix. Kinda checks out. Any thoughts are welcomed. Video

That speed range says to me: Tires.   Check for round and balanced.

I watched the vid.  Sure it's not death-wobble?  Super Duty chassis was known for that.  Easy stuff first, rotate the tires and see if it affects anything.  If it's DW, I've had great luck adding caster.  A lift kit tends to move caster toward the negative.  You need to add it back.

From the video that seems like tires.

I know exactly why. 35" MTs, and 6" lift!

On a serious note, someone put stock size AT tires on the F250 I just bought they were very low miles, on a good condition 2wd truck, and had still cupped so bad they would shake your eye teeth out!

My 79 K30 4wd dually would go into death wobble with street tires and a tight front end. 
While Super Duty's do have a rep for the death wobble, my personal experience is there is always a mechanical failing reason. 
That vibration is (imho) tires. 

I also vote for tires due to the frequency of the vibration.