Truck began to smoke & leak profusely from the bell housing

Background: This truck sat for 5 years in a garage with absolutely no movement. I have serviced everything because of that. The truck runs & drives great under normal driving. I have put 10,000 miles on it since I purchased it & did that full service. Even in town & hills the trams temp never gets over 180 degrees typically running around 145 degrees in 60 degree weather. The other week when trying to pull an unloaded 6x9 utility trailer in mildly hilly area of southwest GA; 10-15 miles down the road from where I borrowed the trailer, the truck began to smoke & leak profusely from the bell housing. I had someone come w fluid & to pull the trailer back. After topping off the fluid & driving similarly as to how I drove with the trailer there was no more leaking. I have since tried to reproduce the problem with a transmission temp gauge installed thinking as light as it is the transmission was overloaded with the trailer. I have always driven it in overdrive; with the temp gauge I verified that it runs cooler in overdrive but builds heat fast when pulling the trailer & it downshifts for slight grades. When attempting the same rolling hills w the trailer in tow everything was fine until powering uphill and trans temp reached 195 degrees at which point the fluid began to pour out of the bell housing again. I know 195 is getting to the high end of acceptable temperatures but I don't think it should pour fluid out at that temp. After topping off the fluid and continuing to drive normally in lesser hills it was fine staying around 180 degrees. At home I verified that there was proper coolant & the radiator & trams fluid cooler were not filled with debris.
Please help?