My company pickup is a '99 4x4 with an auto, and 220,000 miles. From day 1, according to the boss, this thing would heat with a trailer - I've flushed the system, and everything looked perfectly okay, not too terribly dirty, and it didn't seem to make any difference. I'm assuming it's the tranny that's making it heat, what should I use for a tranny cooler? Any other ideas?