95 T100 extended cab 4x4 with A/T shifts out of O/D in head wind

I recently bought this truck, it has 170,000 miles, 32X11.50X15 tires, and a 3" body lift and runs great, but it is constantly down shifting when driving into a head wind.  It feels like a lack of power.  I thought that maybe the tires were too big and I might need to change to lower gears until I found out the gears that the T100 came with are 4.10 or 4.30, so that can't be the problem.  The truck is averaging 14.5 mpg but if the wind is blowing it averages 13.5 mpg.  Could this be a bad tranny.  I've had the tranny oil light come on twice when fighting a strong head wind.  I am installing a transmission oil cooler today to take care of that problem.