Dave,

I think it is a combination of both. It is probably possible for moisture/winter road salt solution to get inside the axle. Looks like it rusted from the inside out until the metal was weak enough to fatigue and break. It appears there is rust coming thru the axle toward the right end also. This material should be thicker. This may also be a clamp point during fabrication or something like that since the rust appears to be somewhat symmetrical end to end.

About any axle you look at has rust on it but they are usually thick enough so corrosion doesn't weaken the axle to the breaking point during its useful life. 90,000 miles and 8 years to structural failure of a safety nature is premature in my opinion. There is very little rust on the rest of the vehicle.

Jim at Dawn