I'm not quite certain what you're describing.

Is it a single "click" .... or more of a "clunk" as the drive train loads up?

Does it make the noise only one time as you begin to move? Does it "click" again as you decelerate and load the suspension in the opposite direction? Or if you turn sharply at low speed? Do you only hear it, or can you feel it in the steering wheel or floorboards?

If you jack up the car in gear, grab the tire and try to rotate it forwards and backwards to see if you can duplicate the noise.

Even though it's a rebuilt axle, it may be flawed. But there are other possibilities. A visual inspection of the CV joints is necessary, if only to eliminate them as a source. And try both sides... you only replaced one of them, right? A single "click" (or something like that) can also be as simple as a strut mount. You need to narrow this down a bit more before you throw any more money at the solution.

Your mechanic may well have been correct... (perhaps the hub and axle were the source at the time they were replaced)... and now it's something different... are you sure it's the same sound?

Good luck

Marty
'96 855T