Should SOA pay for this?
Well, 28,000mi and the rear axle is being replaced? That sounds like abuse. I have 38,000mi in 2 years on my WRX and I have not had a single component replaced (knocks on wood).
Should the dealership pay for this?
I believe that if you are going to sell a used car w/ a warranty then the dealership SHOULD inspect the car for damage and fix, or notify the buyer of the defects. Sell the car As Is if it is in bad condition.
Is the buyer responsible?
Personally, if I ever bought a used car I would have a neutral 3rd party mechanic that knows Subaru's inspect the car before I purchased. So it is your fault for not seeing the issue as much as its the dealers fault for not telling you about the defect.