I've had this happen intermittently since I've owned this truck but right now I think its as bad as its ever been. When I start the truck cold and drive somewhere, my torque converter will lock up in OD the first time I get above ~50 mph. Once it unlocks, it will not lock up again until the next time I drive the truck. This is not a big deal on my daily commute but its killing my mileage on the highway. I got about 17 mpg on my last highway trip where I used to get between 19 and 20.

Anybody had this happen or have any idea what may cause it?

97 T100 4WD SR5 - 231K