Be interested to know how the thermostat works out. It's hard to say if the temp jumps up from being at high rpm for so long and water flowing too fast or if the thermostat isn't allowing enough flow to keep it cool. A cooler thermostat will make the first situation worse. It's unlikely that's the case, but being that the trucks were designed with 90% of it's usage being under 2k rpm (I believe that was the GM stat I heard, maybe it was 2200) then it could be that the stat is too open and water is moving too fast through the radiator to cool down.

Most likely the cooler stat will do the trick, I can't help but to over analyze **** though.