Thanks for the replies, all.
Bobb, I think I had heard that before, but I've actually never encountered any difficulties with shifting to fourth when cold - even when we visited Colorado/Idaho/Utah, where we encountered many sub-zero mornings. That being said, the only time I notice it now is when I'm leaving work, where I leave the parking lot into a 55 mph zone. Most times, it's a short drive through town to get to a highway. Maybe that's the difference.
I'm so confused!
Nick, I'll admit that I don't know much about the process, so while I might have heard someone say backflush somewhere, I really don't know if the shop plans to back or forward flush, nor do I really understand why that might make a difference. So, might you guys just recommend changing the fluid, without back or forward flushing? Or should I just leave well enough alone?
The car's going to the Subaru mechanic next week for a new axle and it's first plug change.
Maybe I'll see what they recommend.