Otrfun is correct. The lower the rear axle ratio, the more stress it puts on the driveline component between the engine and rear axle. This is regardless of trans gear.
Downspeeding has become a big deal in the medium/heavy duty industry in the past ten years due to the new fuel economy regulations for these class of trucks. What people thought they would save in fuel economy, they ended up spending on driveline components because the low rpm and gearing put a lot of torque stress on the driveline. They finally started making drivelines that can handle it, and many handled it great within their respective ratings. However, the drivelines of the taller rear gear still could not handle as much weight as the shorter geared trucks due to this driveline stress.
I have no qualms with using my 3.42 axle to tow 15k, but I would started looking at getting a truck with shorter gears if I towed more than that especially if it was on a regular basis.
