rear-end noise

i have a '96 vette with 125,000 miles (purchased at 55,000).  daily driver.  recently developed a "bearing-type" heavy sound in the rear somewhere.  no vibration with sound.  replaced driveshaft & half shaft U joints, driveshaft harmonic balancer and passenger side rear wheel bearing assembly.  driver side rear wheel bearing appears OK (so did passenger side bearing once removed!). 
 
no change in sound since said repairs.  easiest to hear noise at about 34 mph when not accelerating or decelerating, but it is always in the background at any speed.  shifting to neutral and side loading turns don't appear to make any difference in sound.
 
any ideas anyone??
 
125Kvette

Got 114K on my 94 and it has always had a bit of a "sound" when I would let off the gas. I've replaced half shaft u-joints, checked wheel bearings, etc. Still has the sound persists, I just don't worry about it any more.  I hope someone here can give you some help, or do like me and not worry about it.

Could be one of the carrier bearings in the differential as they tend to make noise all the time as opposed to pinion bearings that usually make noise when you let off the throttle.

Thanks L.B.  Wondered about carrier bearings myself.  Didn't think the differential would be the problem as the drivetrain is designed to be pretty tough and I drive the car like you'd drive a normal sedan most of the time. 
 
Suggestions on when to consider differential R&R??  Is sound anything of an indicator?
 
D.C.

Might oughta drain the fluid and check for metal in gear lube.That will be the tell tale sign.