Knocking in rear after work on differential
 

I have a 2004 Mountaineer, the oil leaked out of the differential & I ended up having to replace the gears, bearings, shims, clutches in the differential. It now makes a loud, continuous knocking sound while driving on deceleration only (letting off the gas).

Have taken it out and had someone "re-align" everything and put it back in, still having the same loud knocking (this is new noise, was not there before work was done on differential). Does anyone have any idea what this might be? Thanks-