Mackinaw, if this is the first time it's done it, and after it cooled down it quick leaking your probably fine. I had the same happen to me a few years ago on my 2003 F250 V10 4R100 powered truck. Towed 150 miles into the campground, stopped to unhook the boat, pulled forward and had to back up a 5-7% hill to get into my spot, and of course it took me 2-3 tries to get it right where I wanted. Got all done, looked down, and sure enough I had trans fluid dripping just as you experienced.
That evening I called a good friend whom is one of the most reputable local trans builders and had a chat with him. He said not to worry about it, more than likely it was just the front seal leaking and when it cooled it would likely stop. His explanation was; basically your trans was at full temp when you pulled into the campground, you then proceed to back uphill a coupe times, and you then had very little air going thru the trans cooler. In reverse your torque converter won't lock under those circumstances and you'll overheat the fluid around the torque converter, this causes the front seal to get soft, expand a bit, and starts leaking until it cools down. He even asked if I noticed if my gauge moved and told him that I didn't see it move which he noted that in this case often times it won't show on the gauge since only that fluid right around the torque converter is what is overheated unless you continue to push it. He did tell me that since mine was 4 wheel drive, that if your maneuvering heavy trailers at slow speeds, lock it in four low and it'll take enough stress off the trans that it'll keep it from doing the same in most cases.
He was spot on with everything he said. I ended up taking mine in that summer anyway and had the trans freshened, a HD torque converter, and new seal installed and 4.56s gears put in front and back diffs. Frankly, it made for a good excuse to the wife to get my truck bullet proofed like I wanted, ha ha. Truck had about 65k on it at the time and was 7 years old. The guy at the shop said all of the original parts, gears, clutch plates, everything looked like brand new. Heck they even joked that one of the younger guys wanted to save the fluid for him self. Point being, the trans was fine and still in great shape even after my small incident.
Good luck, if nothing else it can be used as a good excuse with the wife that you "need" to get it fixed and that you should probably upgrade a few things so it doesn't happen again. Good luck
