Sound from differential
Post 2 had the right answer. (95%)
One or both of the u-joints are worn.
Not to say that it wont last a while longer, maybe a long while, and maybe not so long.
It is very hard to see this kind of looseness in a u-joint by shaking or prying on it, it can be found by getting under the vehicle while someone that you TRUST A LOT sits in the drivers seat with his foot firmly on the brake while shifting from reverse to drive repeatedly.
I would NOT do this with a vehicle on ramps, stands, or a jack.
The fact that I am opinionated does not presuppose that I am wrong......