Subaru - Forester :: Thumping Noise When Accelerator Depressed At A Speed Over 35 Mph

I brought my car to a dealership for its 120,000mi check-up, and they told me that the front axle boots were torn. They replaced the right axle, and on the ride home, I noticed a burning smell. Short story is four return trips, and the right axle replaced three times (including the initial replacement), and the smell has finally gone away. The weird part is not that I went back to the same place four times instead of giving up, it's that they told me this last time that they moved the replacement #2 right axle to the left when they installed replacement #3.

Why would they do that? Why would the smell go away if the issue was with that axle? I looked it up, and the same part is used on the driver's side and passenger side, so it wasn't as though they installed a left axle on the right the first two times. Does this mean the "new" axles were just new-to-me and had been reconditioned? If reconditioned, would it matter if a previously left axle was mounted on the right?

Meanwhile, about six weeks after replacement #2, the car started making a thumping noise when the accelerator was depressed at a speed over 35mph (but not when coasting above 35mph or with the gas on below 30-35). I was wondering if it was misfiring or something, but lo and behold! That noise went away with the axle switching. Way back at replacement #1, they replaced the boot on the left but not the entire axle until #3.