Sorry you haven't found a solution yet!
We live in Houston, Texas-it gets a little hotter than normal here :). So, the heat factor could make sense.
I am no motor head, either-by any means. I quit my job about a year ago and money is a little tighter, so it made me a better investigator. I no longer just drop it off at the shop with little to no idea what is wrong. I nudge around on the Internet until I, at the very least, have a better understanding of the problem (or the possible problem).
I'm dropping it off tomorrow at I place I trust (ie-they haven't been dishonest yet), and I will let you know what they say (maybe it will help you figure out what you need to do for your Subaru).
I may try to get under the car tonight to see if I can see the rear differential, but all the diagram/photos I've seen so far-it doesn't look like you can actually see the fluid-you have to unscrew some stuff and let it drain. So, I don't know how to tell if the fluid is actually low or not? Back to the Internet!
Thanks for replying and I will let you know what I find out!