Truck feels as if it drops into neutral while driving

The other night, I was driving home from the store maybe a block away from home, all of a sudden the truck just started smoking. The quick connect on the heater core broke off and started to shoot coolant all over the passenger side of the motor. I replaced the quick connect, bled the cooling system of the air bubbles let it run for about a half hour forty five minutes with no signs of over heating or anything. I am not getting any back pressure through the reservoir tank, no white smoke from the exhaust. I checked the transmission fluid, its a little dirty but does not seem to be low at all. Once I took it for a drive it was shifting okay until the truck got hot. After about twenty to thirty minutes of driving it started to feel like the truck was all dropping into neutral while driving at highway speeds. It seems to get worse the more I drive it and the hotter it gets. I was just wondering if someone could give me an idea of what it could be? I have had a few people tell me that the transmission is starting to slip and eventually the transmission will need to be replaced or rebuilt.
Keep in mind the vehicles age and miles on the body, the engine, front and rear differential, were all rebuilt about 75,000 miles ago.