Still have low freq vibration (95 A6Q)

[beattyr2003 at cox.net]

I posted about this oh about a week ago and only saw a couple responses, neither of which could be the problem.

I have noticed some more things about the symptoms.

Its most noticeable at speeds around 20-40mph.  Its irrelevant to having the car in gear or in neutral, clutch in or out.  I had said before it went away when I turn, thats wrong, it doesnt go away turning it stays the same.

The best way I can think to describe it is if you were driving on a very finely cut rumble strip in the road.  Or grooved pavement.  It also feels/sounds? like its coming from the rear of the car.

Since its not dependant on the engine being engaged, I can rule out anything motor related.

I know what wheel bearings going out usually sound like, they howl at nearly all speeds instead of vanishing at 50+ mph.  So that rules out bearings.

I am left with thinking its either something in the rear drivetrain, or my tires somehow have worn in an odd fashion to cause this odd noise. 

Anyone got any ideas on this?

Things I have done with the car recently are replacing the prop shaft rear CV joint over the summer and I just replaced both rear tie rods.  I have verified that the center bearing support has not come loose nor does it look like its in terrible shape.  I could crawl back under the car and verify that my CV joint bolts havn't started loosening up, but other than that or swapping all 4 tires out, I am kind of at a loss for where to go next.

Thanks,

Rob
1995 A6Q (271k miles)