94 astro Please help weird problems

Ok here is the deal with this very troubling problem I am having.

Astro Van '94
180,000 miles
2WD
When I am at a speed in excess of 50mph this problem accurs.

At any speed above 50MPH when I take my foot of the gas and the vehicle begins to slow down I get a low pitched rumbling sound that sounds like it is coming from the rear of the car. It also feels as though something it's holding the vehicle back and not letting it run as smoothly as normal. Once I hit 80MPH the vibration and sound is constant when my foot is on the gas. The sound and vibration only occurs for the first 5mph of decceleration. So if I am doing 70mph and take my foot off the gas I get a low pitched rumbling sound (rumbling, grinding, mechanical type sound), a slight vibration, and it feels as though the vehicle is hesitating.

So far I have:
Changed the universal joints.
Checked the rearend and the fluid seemed good (I replaced it) and there was no metal shavings at all in the fluid, or on the magnet.
Checked the trans, fluid seemed alittle burnt, replaced the fluid and fluid filter, gasket all that good stuff.
Had the tires rebalanced.

Problem is still there and I can't figure it out. Neither can anyone I have talked to already.

Has anyone has this problem? Anyone have a clue as to what it is? Help.......

Sounds like an angle problem to me. Something is allowing the angle between the trans and differential to change enough to cause a bind and thus the vibration that you are feeling.

Could a rear transmission mount could be weak rear springs, a broken mount for the rear-end to the springs....

I should look for? Thank you for the suggestion. I'll check it out sunday. or maybe tomorrow if I have time.

Make sure the pinion nut on the diff. is tight.