The fact that it only happens on deceleration leads me to believe that it's an issue in the rear end, like when you unload the drive side of the gear teeth and it loads up on the coast side, something isn't happy. Did you change out the ring and pinion when you had your rear end torn apart? Is there friction modifier in the gear oil?
If you're the handy type, you can crack your rear end open and check the gear teeth, but keep in mind you'll need a new gasket, a couple quarts of gear oil, and a bottle of friction modifier to get it back together. Also you won't be able to dump gear oil straight into the filler hole so get one of those cheap bottle pumps if you don't have one.
My dad had a somewhat similar issue with his '05 GT at around 140-150k miles. He fought with the rear end for months until finally ending up with a whole new axle. Hopefully it ends up that I'm just talking out of my ass here and yours ends up being a simple fix.