All. I have a high frequency vibration at 75 miles and hour and up. I am attempting to diagnose it. Here is the list of my details.

2013 E350 front lifted with BFG AT2's.
2" rear wheel spacers for stability
5.4 V8.
No rear lift but towing helper springs.
No seats or cargo in the back.
When this vibration started I had just installed a 4.53(?) from a 3.9 rear end gear ratio to tow better.

Here is what is happening. I am getting a slightly noticeable vibration at 75. When you are above 75 and decelerate it almost gets worse and then goes away again when dropping below 75.

This is what I have checked.
Wheels and tires road forced balanced. No difference.
Had the new diff Checked.
Had the drive shaft checked again, considered fine.

Things that concern me.
I am getting some brake rotor wear/warping that I am going to address this weekend.
I was getting loud rocking ping noise when putting it in park on an incline prior to the diff change.
Guys put it on a lift and rolled the speed up. Said that the wheels hopped and such pretty bad at speed on a lift?????????

Where would you guys recommend starting to go from cheapest to highest in cost to get ride of this. I want to be able to drive 80 on the freeways.

Thanks.

RickM