My RS has cause a cold!

Hey all,
My Impreza has been acting funny these past cold days and I wanted to get some
input from you all before I take it to Flatirons Subaru where they like to tell me they don't hear anything. I have been hearing a distinct low grinding sound coming from my gearbox. When I change gears in any manor but perfect, the rear axle will "buck" making a "thump" sound like something is sliding around in my
trunk. (Yes, I checked my trunk to make sure nothing was sliding around) And
yesterday when I was coming to a stop at a light with the car in gear the rear-
end started shaking quite badly, the road was clear and smooth. The odd thing
is today in the afternoon when the weather got warmer it all stopped. The
reason I ask you all is because I don't want to be told I wasn't hearing
anything again. What should I tell them to check for? I just had my 30,000
mile service done (new trans/diff fluid and flushed radiator--But done at Jiffy

Lube) I really appreciate any help and advice anyone has, especially if some
of you other guys with Imprezas have had the same problem. Do you all think my

car just doens't like the cold ?? ) THANKS!!

Scott