Strange vibration at certain speeds

2003 Avy with 4wd Z71 package, 5.3L engine.

When going down the highway, a noise and vibration start that I can only describe as it sounds like my tires are trying to saw through the concrete starting at 55mph.  There is a definite tire noise and the whole truck starts to vibrate.
The problem gets worse as I accelerate and is at its worst at 65mph.  Continuing to 70mph the noise and vibration fall back to "normal" or about what you would expect at that speed - virtually none.

There is no extra vibration in the steering wheel - like when you have a tire out of balance.  There is no pull to the left or right like a bad alignment.  It seems to come from the front and rear equally, but I can't be positive of that as the whole truck seems to be vibrating equally.
The problem exists when accelerating, decelerating, or holding a steady throttle.  Braking does not seem to have any affect either.  I usually drive in 2wd, but the problem happens in auto as well.  I have not tested it in 4wd, but I suspect it would not affect the problem.

I thought doing a tire rotation might help, so I moved the rear tires straight to the front, and crossed the front tires to the rear. 
When changing the tires, I looked closely for uneven wear, or problems in the tread pattern, but everything looks fine.  The rear tires (now on the front) are at about 50% tread.  The front tires (now on the rear) are about 70% tread.

The problem still exists and might actually be a little worse.


Any ideas?  Would a bad drive shaft cause this?  Could it be a camber/toe problem?

Bad u-joint.

Did you get tires BALANCED?

As tread wears away the original balance job does not always help as they can go out of balance.

Bad u-joint, bent rim, damaged tire or other suspension component.

I brought it in to my local tire shop and they spun the wheels in the air.  Reported that they were "hopping" all over and that these tires (Goodyear wrangler authority A/T) were known to start coming apart inside when they get near end of life.
So I pulled the trigger on 4 new General Grabber AT/2 tires.  They said my front end components looked good and that rims looked fine.
I picked up the truck and she rolled smooth as butter around town. 
However once I headed home and got on the highway, I felt the vibrations starting at 55mph and got worse as I increased speed.  Again at 65mph they were terrible and loud.  Not as bad as the old tires, but still vibrating so bad that when I looked in the driver's side mirror, the view was blurry.  It is a very fast vibration - if you can imagine feeling every tread bump at 65mph.  At 70mph it smooths out again.

So I think I can eliminate the tires and balance.
I called the shop as I was driving it home and I'm scheduled to bring it back in on Thursday.
Will update then.

have them check your wheel bearings and rotors for warping while in there!

Didit

Just an update.
The vibration turned out to be a bad u-joint at the rear of the drive shaft.
New u-joint and it's smooth as ever now.

Glad you got it fixed, my AV was doing the exact same thing at 65-70 it was fine.

Getting up to 65 was quite a noisily ride.