More of a concern than a problem but, I have a noise,best described as as moaning sound,coming from the rear of my car under deceleration. Its noticeably at speeds over 40 or so but only when im off the gas. At a steady speed it seems to quiet down and i dont notice any other vibrations. The diff is a 12 bolt with a posi in 70 Chevelle wagon. Thanks!

Sounds like many of my cars from my youth - worn out. I have always associated a howl/whine/moan in the diff. especially on decelleration as an indication of it being worn. It probably will go many thousands more miles, but the noise may get progressively louder. Have you checked the fluid for level and or metal flakes?

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That's your gears in the diff whining because they aren't meshing together like they should. Most likely the gears are shot due to some things you better check soon.
Loose pinion bearing..
No rear diff oil.
Incorrect gear install.

Have you had this rear a long time or just did a gear change??? Can you be more detailed.

Picked up the car about a month ago already built so im not sure whats been done so far. The sad part is i bought it from a guy who runs a rear end shop. Going to pull the cover off sometime this week and take a look in there.

Unhook the driveshaft and see if the pinion yoke feels loose by pushing it front to back.
If it is, then the pinion preload wasn't done correctly. When you pull the cover, check the gear meshing, or contact pattern to see if it's correct. You could just have a bad wheel bearing..