I was driving down I-24 pulling a U-Haul trailer the other day when my '97 80 overheated. I had driven from Minnesota to Tennessee with the trailer and hadn't really had any problems. However, going up this really long mountain pass, the truck just kept getting hotter. I was going about 25mph and had turned the heater on. Pulled of to a rest stop to let it cool, but it would no longer cool down at all and would just steadily heat up again. It wasn't leaking any fluid, though it did have steam coming out of it somewhere in the rear, driver's side area. All the belts were fine and the radiator didn't explode. I checked the level before I left Minnesota and twice along the way and it was fine. Any ideas what could have happened?