Tires: Rear end Humming vibration only @/about 120 Km/Hr ., tire rims, nissan pathfinder

Question
Hello Barry,
That vibration comes on and off since new, but after i changed my original tire/rims to an after market set, the vibration is now all the time when the speed at or about 120 K/Hr., some times "5%" it only appears @ 130-140 K/hr.,
checked tires/rims "old and new tire/rim packages", dynamic balanced them allignment done 'old and new tire/rim packages'
New brake rotors , pads, rear shoes, drum service, front bearings changed, rear axil bearings checked by the local dealer.still that vibration appears as per above.
For a 2003 Nissan pathfinder LE 4WD
original tire/rim specs. are :-
245/65/17 97H tires @ 33 psi on 8.0" X 17"  +20 mm offset rims.
New Tire/rims specs.
255/45/20 105V tires @ 39 psi on 8.5" X 20"   +9  mm offset rims.
For a Difference in circumference of -40.21mm or -1.71%
Thus (the inside of the wheel to the strut housing)will be more by 5mm  and (position of the outside edge of the wheel)
will be extended an extra 17mm. and the SUV will set lower on the ground by 6.39 mm.
My questions:-
1-The cause of vibration ?
2-Why the new set up made the vibration occurs most of the time? why the trigger speed sometimes goes up ?
3- the tire geometry results i mentioned above were derived from an online calculators and do not make sense to me as the +ve. offset reduction by 11 mm and tire section/tread width increase by 10 mm would not make the tire extend out "one side by 17mm? or dose it ?
thank you for your time and efforts on this one,
Cheers,  

Answer
Hissein,

1)  There are so many rotating components that it is hard to diagnose over the inernet - and as you have already seen, in person if the problem isn't obvious.  

What I think it might have been at the beginning was two things that were going in sync and out of sync - and I suspect a tire / rim assembly and a front axle assembly.

Now that you have changed the tire and rim assemblies, the only problem left is the axle assembly, which isn't being cancelled by the tire / rim assembly - so it is there all the time.

But I don't want to rule out some other rotating component.

2)  I think I partiallly answered that question except for the part about why the speed went up - and I think that might be due to the road surface causing a vibration that is cancelling the lower speed, but in sympathy with the vehicle at the higher speed.

3)   Well, I don't think that line of thinking is getting you anywhere as far as the vibration is concerned.