My car had been sitting under the hot sun for 5 hours today (92 degrees F) and as I took off from the parking lot I noticed rear differential temperature light come on in 3rd gear. I immediately pull over and give it several min to cool off and take off again, and the light keeps coming back when I hit ~2500 rpm in each gear. I tried my best to stay under 2000 on my way home and now the light comes on past ~3000 rpm (it's now 80 degrees F outside).. Could this be simply due to not having enough fluid? (I've never checked the rear diff. fluid) Or will I have to take the car to a dealer?

You don't say how many miles you've driven in your STi but given the fact that my 05 with 13500 miles has been out all day in equally hot weather and never had a temperature issue I would guess you have either lost fluid or have a faulty sensor.

Either way, a trip to the dealer sounds likely to get it sorted out.

David