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You can sure feel like you’re chasing ghosts when you’re on the trail of some irritating noise. It does sound like you’re building a list of things which are not the cause. Assuming high confidence in those pros with the lift, keep on that list anything they had a good look at, that is, the entire drive train. Unless what’s behind this noise is a really exotic problem--and that’s unlikely--they probably would’ve caught it. The usual suspects those pros knows.
As far as you rear axles: again, they probably belong on the not-the-problem list, as does the rest of the rear end. What fails when you have noisy axle-shaft trouble is the axle-shaft bearing. As it fails, the bearing begins to roll less and less well, with the rolling ultimately becoming a grinding. Failed bearings will announce themselves. Although the noise of a failed bearing will change with load, it is not an intermittent noise but a continuous one. So far, you have not described such a noise.
If you do pursue your ghost to the area of the axle shafts, this might help a little: You can get the rear calipers clear of the rotors without opening the hydraulic brake lines, meaning you won’t have to bleed your brakes. The steel brake lines are attached with bolts and clamps to the axle tubes of the rear end; loosen those attachments and you’ll have enough movement to get the calipers clear. Then, suspend the calipers with some stiff wire secured somewhere on the underside of the car--I think the upper shock mount will work.
The problem with these undercar ghost noises is that it’s hard to locate the source when you’re in the driver’s seat, and it’s hard to demonstrate the noise when you’re not in the driver’s seat. Also, suspension components behave differently--and may make different noise or no noise--when they're “unloaded” on the rack compared to when they’re “loaded” on the ground.
Maybe you could enlist your extra pair of hands to rock the car back-and-forth and side-to-side while you sit at the wheel with the engine off. Brakes on and off, parking brake on and off, in and out of gear. If you could get the noise to repeat on demand, then you could switch places with your assistant, get on the ground and trace the noise to its source.
You might consider whether the ghost noise appeared after an apparently-unrelated recent repair. Just because we don’t have any parts left over when we’re done doesn’t mean we got ‘em all back where they belong. Have you recently done work on the rear brakes? The parking brakes can be tough to get back together. Assuming that you use your parking brake, maybe you’ve got a spring or a cable hanging up in there which prevents those little drum brakes from releasing smoothly. If they are hanging up, they could easily make a pop, ding, clang or clunk when they finally let go.
Good luck with it!
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