Engine/Transmission (1998.5 - 2002) white smoke vents

[Kf6cmp]

Hey all, my truck is driving fine. All running smooth and power. But after my truck warms up and going up hill and/or throttling it hard; White smoke comes out of the vents in the truck. Its intermittent and seems to dissipate until I go uphill with more power needed. There is plenty of radiator fluid in the over flow and the truck seems to run at regular temp. No leakage detected in the cab for a leaking heater core. Any ideas. I'm thinking a problem with thermostat. Chris