If you run steel tubing it will help in cooling, as rubber will not! The only downfall of a rear radiator is to make sure you have sufficient airflow in the bed!
Why don't you get a front skidplate, that will solve your problems! You'll still get airflow through your radiator, but you will keep stuff from damaging it! Get some metal or aluminum and have it bent to fit from the front cross member to just below your bumper!
