I took off the filler plug and OUT IT CAME! A fountain of fluid comes pouring out, lucky my pan was there and there was only minimal spillage. Drained the rest from the drain plug and measured it out; 4.75 qts. I am assuming 2 qts GL-5 and 2.75 qts ATF. I changed my tranny fluid about 3 months ago and have checked periodically since, no loss of fluid. I can only assume this is from a long time ago. After i changed to nice new fluid, it feels as if 500lbs have been lifted from the truck. It is quicker, it shifts better, and the hum that i was getting in 4th gear is now a nice little whine. Can anyone explain what happened? How did all that tranny fluid get in there?