I have a 700r4 that was rebuilt by bowtie overdrive about 1 year ago and it runs really hot when going up, some what long steep grades. It runs great the rest of the time even when offroading and all other conditions. It will heat up in any gear not just overdrive, plus I do have a aftermarket tranny cooler. 

. Its running through a new radiator cooler and a aftermarket cooler.