EVERETT - It's possible that the hydro charge pump may be cavitating, the Hy-tran in the rearend could be too thick to flow into the oil intake and at the higher RPM it's starving for oil.
But normally Hy-Tran won't do that until it's colder than just Freezing, 32 deg. unless it has a lot of water absorbed into it. Has Your Hy-Tran & filter been changed?
Hydro's put a lot of heat into the oil, letting it set idling at 1200 RPM or moving around slowly at that speed should help the oil warm quicker and minimize the problem.
When it gets cold I only ever run stuff I have to run. Wife gets a little irritated sometimes because I don't get the Super H & loader out to push a half inch of snow so She can get out with Her AWD SUV. But when the Super H does come out it gets fully warmed up, well, at least the engine does. Me and the whole rearend freeze to death!