Help - Buzzing/Vibration at 20-50MPH???

I am curious - as to why I am having a buzzing to vibration?

I have a 2006 TRD DC/SB, 3in lift. New Tire & Wheels was put on a week before the lift. So I know it is not the Tires making the noise.

I recently put on Bil 5100's @ .85, Eibach Springs, 3-leaf AAL, Diff Spacer, C/B Drop, and Camburg UCA's. ( control arms was just done, and tire rotation. )
I thought it may have been the cause at 1st! because the Caster n Camber - was out of Spec.

But every time I start off and reach about 20mph, and until 50mph. I have this noise. under or over those speeds it is quiet!! Also doesn't seem to matter what type of road I am on, and if I sway my truck left to right. the pitch or sound doesn't not change. so it doesn't seem to be a hub bearing or so!!!

THE ONLY WAY I CAN REDUCE THE BUZZING OR VIBRATION -
is to put my truck into 4 wheel drive!!! then it is barely even there!!!


Alignment Specs -

Toe - .0-2
Camber - .4-5
Caster - 2.7

Tires are Stock Size, 265/70/R16 BFG's Rugged Terrain's

I don't know what to do next?
So if anyone, knows what it is, or could be?

Please let me know, Thank you...