These ads from GMC always give me a kick: “Never send a truck to do a Sierra’s job.” Take a gander at this one:

It may not be that you use your truck to tow other trucks a lot, but this ad does have a very realistic scenario: towing in the mud.

And the “selling point” of the Sierra here is the locking rear differential. Normally a rear differential allows the wheels to spin at different rates, as when you are cornering and the tires actually “want” to travel at different speeds. This prevents tire wear and is a good thing–most of the time.

But imagine now that you’re towing in the mud. A non-locking rear differential allows the torque to go to either wheel regardless of whether its gripping or not. You just end up spinning and flailing about in the mud.

A true locking differential prevents this mess. As you move forward slowly in the mud, giving your Sierra a little gas at a time, you’ll be able to “feel” how the tires are gripping and steer appropriately because the same amount of torque is going to both wheels. No flailing.

As the ad indicates, anything but a true locker is going to be “crud in the mud.” Time to get a real truck!

The Wikipedia article on locking differentials has some good info. Check it out!

–Truck Man