"Clunk" from Drive to Reverse, etc.

Mechanic did temporary fix, "metal glue", I believe it's a type of threadlock or loctite product. Held about 30,000 miles or so.

Told me about 5,000 miles ago it failed on the transaxle side. I know nothing about half-axles, or how they fail.

Do the splines strip on the hub or transaxle side? I'm going off on a tangent here, but if it stripped how do I know the hub or the transaxle has not stripped out? Is there a mini-universal joint in the assembly that fails? Are there shops out there that take the axle out, repair it, and put it back?

If he glued it, does that mean there's going to be some left over glue inside the hub or transaxle? Am I supposed to used metal glue when I put in a new one? In a refurbishment axle, exactly what do they replace?

I'm a little in the dark here, because I lack experience. Should the replace the transaxle seal? Is it straightforward, like a camshaft seal, that you carefully pick out and tap in the replacement?
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I'm stuck on Volvo and Volvo's stuck on me....