b/c of the load, most "normal" sized vehicles with an auto trans won't be able to lock the torque converter in OD. The TQ produces a lot of heat when unlocked. Coolers are not expensive to buy and if you put it in yourself you can probably be done with ~$50.

Now, my F350 diesel tows just about anything in OD, but... i do have 600+ ft-lbs of torque and an aux cooler