2000 GMC Sierra Overheat

[cardriver]

I had my truck overheat on me recently.....

I had woken up on a Saturday morning, with an 18ft. trailer loaded with a four wheeler and a Suzuki Samurai. I pulled it about 50 miles between 8am - 10am.. 90% on the highway with O/D locked out. I ALWAYS lock out O/D when pulling anything with a load.

Everything was fine until I decided to make my trip home. About halfway back, around 3pm, I noticed my temp had risen closely to 260 (normally runs around 190-210). I was doing around 70mph, without much downshifiting on inclines (not very steep, just enough to slow you down). So I dropped my speed down to around 55-60, and turned off the A/C. This lowered my temp down about 20 degrees, but it was still running warmer than usual.

I stopped at the next gas station I could find, and checked my coolant level. Everything seemed good. Getting back on the road, I noticed since being completely stopped, my temp was back to normal, but again rose as I got back on the highway. I ran the rest of the way home with no A/C.

I noticed when I got home, that there were oily spots on the front of the trailer. I looked under my truck and did see my crossmember had quite a bit of fluid on it, and traced the leak back to what appeared to be an overflow hose.

My Questions:

1. Did the transmission start to overheat, and cause my truck to also overheat? Could this have caused any problems I should expect to see?
2. Could the fluid from my transmission have gotten hot, and somehow boiled out.
3. How did turning off the A/C reduce the engine temp?

Any help on this matter would be greatly appreciated. Thanks in advance!

Charley