95 AWD Astro Vibration

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.(big$- 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

Have you changed the transfer case oil recently? Did you use the new formulation Auto-Trak II oil? It was designed to reduce transfer case shudder, pursuant to the TSB for this problem (it's a synthetic oil).

You also need to be sure all the tires are the same size and SPEC number, and are properly inflated. Inspect the tires for signs of belts slipping (bulging sidewalls, deformed tread pattern), and possible tread/cap separation. While spinning the tires to inspect them, see that the wheels are not obviously bent or loose.

Next, you can start looking at the driveshafts for loose flanges, CV and U joints, check for eveidence of a spot welded balance weight being broken off, and look for bending/straightness of the shafts. Since the front prop shaft was removed, look for evidence that the flanges were marked to retain correct position/orientation, and therefore correct balance.

Beyond that, you would need to look at control arm bushings and anything that might affect camber and toe of the front wheels.

Thanks for the reply. I have had the tires all replaced and alloy rims checked and then ballanced.
I've had the front end serviced and replaced the idler arms with moog's and then re-allighned.
I dont see any weight on the new front propeller shaft, or any evidence of ever being one,,,,hummm,,
I have had all the fluids changed in everything. I don't know if the mechanic used the new formulation Auto-Trak II oil in the transfer case. ,,,hummm,,
Looks like I need to talk to the mechanic about the shaft weight and the transfer case huh? (especially concidering I never had the problem before the original propeller shaft went bad)
Thanks, Doug