My 04 CC 4x4 now has 29000 miles on it. I took it in and had all the fluids changed for the 30000 miles service. After a 200 mile trip home from a mountain trip, I found a large (bigger than my open hand) spot of fluid on the driveway under the rear diff. I called to take it back to the dealership. In the meantime it has not leaked again. The dealership tells me that they cannot find a leak nor reproduce the leak. I feel confident it was coming from the rear pinion seal, as the yoke appeared wet when it first happened. When I went in to pick it up, they had left it on the lift for discussion. While everything appeared dry, there was some caked grease and oil on the gas tank in the radius of the pinion and driveshaft yoke. A different mechanic at the shop believed that may be spray from the pinion, (as the ujoints are not serviceable; thus not likely the culprit). They told me that they would replace the pinion seal as a preventive measure. Does this sound reasonable? As the truck has been leak free since I've owned it, I don't want to do any unecessary work on it. Searching the posts on this site has me paranoid about leaks from the rear. Thanks for your time and thoughts.