[Big Dave]

If you put a serious gear in it (like a 3.08) it slows the torque converter down and reduces the amount of fluid pumped to the cooler. Heat buildup causes spectacular failures, scattering parts down the road. It is a common problem with diesels where the engine is capable of high torque/low RPM operation for hours on end. Heat buildup cooks car-based automatics like the one in a F-150. Check out Ford diesel sites like thedieselstop.com and read the tales of woe among automatic guys who tow a lot.