[LONGHAIR]

Yeah, assuming that it had the correct nut, they do not "come loose". Depending upon the rear end in question, they have either a "crimped style" prevailing torque nut, or a "staked style" nut. These have a thin flange around the threads that gets driven into a groove that is cut into the pinion threads.
Neither of these will loosen on their own, but things can wear internally enough for the pinion to become loose with out the nut ever moving.

As you will soon find out from the mechanic, once the pinion "moves", it is toast. You cannot just tighten it back up. The incorrect wear has already happened. It will howl/whine, which is the sign of it's impending death.

It may be more cost effective to replace the entire rear end with a used one from the boneyard, but there is some work there too. You have to find one with the same gear ratio (or take this opportunity to change to a more desirable one?)
You also have to consider the brakes. The used ones may need rebuilt, or swapped for your originals.
And, you are not likely to get much of a warranty with used parts. If you do, it will be parts only, no labor.