Low Rumble

I noticed a slight rumble in my 05 Prius (120,000 miles) a few months about. I start noticing the noise around 40mph and at high speeds(65-70ish) it goes away. While poking around tonight I found that the back drivers tire is wore a lot more than the right. Also I wiggled the drivers inside CV joint and it moves in and out. Also there seemed to be a little "gunk" built up around it. I'm taking it in this weekend for a transmission fluid change and going to have them look at it. Any ideas so I know what to expect? What is the worst thing I should expect$$$$?

 

I just had new tires put on a year ago (roughly 20,000 miles). What do you think about the play in the cv joint. Is it normal to be able to move it side by side about 1/4 to 1/2 inch? Thats what really scared me. Also the black wetish stuff around the cv joint? There are no puddles or anything just enough to get the tip of my finger black.