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A lot of it depends on whether it make the same noise all the time, only while decelerating, or only under load. If it is quiet under load but whines when unloading for deceleration then you need to check for a loose pinion nut or a bad pinion bearing. When a bearing wears out, the pinion will not stay in position and it starts moving around. While loaded it will stay in mesh but with poor backlash. When decelerating the pinion moves out of normal contact with the ring and starts making noise.
A really bad pinion bearing will allow the input to move when you shake it and it will have no preload and be able to chuck in and out. If this lasts too long, it will ruin the gear set and eventually the housing.
If it makes noise all the time then it could be axle bearings taking a dump or even differential carrier bearings going bad. There is not a lot more that can happen in there other than spider gear failure of something of that nature but it would make all sorts of racket in a turn.