Chevrolet Repair: 95 AWD Astro Vibration, propeller shaft, vibration noise

Question
My astro has a strange vibration and low pitched noise that starts at about 50 mph. It seems to be less severe when I first drive it (cold)  but gets worse after drivng a while.(warm) I noticed especially when I'm driving express way speed (65) with the cruise set, the vibration lessens when climbing an uphill grade. At the top of the hill, when the cruise backs off on the throttle (no load on engine) the vibration/noise increases greatly.When going down hill (when the engine is braking) the vibration is reduced. Also if I disingage the cruise and take my foot off the accelerator the vibration /noise is reduced.The problem is at it's worst when the engine/drivetrain is at a "no load"/"free play" situation.
It does the same thing even when not using the cruise.The
The vibration seems to be in the drive train. I can feel it in the floor of the van and to a lesser degree in the steering wheel. It also vibrates the outside rear view mirror so bad I can't even recognise a car behind me! It's difficult to have a conversation with passengers, especially in the back because of the "rumble"!
I have had the rear U joints replace once  and the front propeller shaft replaced twice.([B]big$[/B]- the first one destroyed itself after driving 25 miles) It still does it. I think it's the front propeller shaft but the mechanic insists it's not.( I get the feeling he just dosent know)
Is there a simple, low cost way to check the balance of the front propeller shaft?
Could the problem be something else? (transfer case,front/rear differential, ETC.)
Any help would be greatly appreciated!
I got to get this fixed! It's driving me crazy!
Cadilad

Answer
The cheapest way to eliminate the front shaft as the culprit, is to remove it completely, and go for a ride.
Remove all clamps and bolts.
It will run fine without it, and if the vibration is gone, your vibration was in it.
If the vibration is still there, more searching is required.
I would have that vehicle up on a rack, and operate it there, while carefully checking underneath.
If the vibration happens to be tire tread caused, it won't happen on the rack, or jack stands.

Van