Just a heads up warning when towing heavy with gas or diesel with automatic transmission on long, slow, steep back country mountain roads or trails. At 5 to 10 mph, the tranny will be in first gear torque convertor mode (not in lockup) and the continous normal torque convertor slippage can overheat the tranny fluid quickly. On very slow speed pulls on gravel or dirt roads, I would use 4wd low range to let the tranny shift up to second gear convertor lockup mode and it would run much cooler. Later traded Dodge 3500 6.7l diesel/automatic for same truck with 6 speed manual transmission, that problem solved.
