wisdom needed...

[triophan]

I have an intermittent (sp?) hum.

I've searched and read and read and searched.
I've got new tires (balanced and checked by another shop) but it was happening with previous wheels and tires too
clean rallys
new front bearings
new ball joints
new 700r4 (well... less than 10K)
new cab bushings
new sway bar bushings
new trans mount
new brakes
just lowered 5" (happening BEFORE lowered)
recent alignment
new shocks all around

I have this hum that I begin to notice about 40mph (incorrect reading on speedo- but some reference point) thats at its worst around 55-60 mph.
I am replacing the u joints and balancing the shaft this coming week...
I guess my question is, is there anything in the rear end that would cause this?
Anything I should check to see if there is any play in my rear?
Could it be the frame being outta whack somewhere?
I really could use some input here. I'm trying to get this dang truck to go down the road smoothly FIRST and foremost.

Thanks in advance.
-david
here's a photo for good measure