Some models are equipped with a locking differential rear axle. If you're not sure which one is in your truck, block the front wheels and jack up the rear of the truck. With the transmission in Neutral, spin one of the rear wheels in a forward motion with your hands. If the other wheel travels in the same direction, it is a locking differential.
DETERMINING AXLE RATIO
Axle ratios available in these trucks are 2.76:1, 3.07:1, 3.4:1, 3.73:1, and 4.11:1. The 4.11:1 ratio was not available with four wheel drive until 1973; the 3.40:1 ratio was available only on 1973-75 two wheel drive models.
The ratios given here are for rear axles; front axle ratios are the same or nearly so. (The 4.11:1 front axle ratio is actually 4.09:1.)
The axle ratio is obtained by dividing the number of teeth on the ring gear. It is always expressed as a proportion and is a simple expression of gear speed reduction and torque multiplication.
To find an unknown ratio, make a chalk mark on a tire and on the driveshaft. Move the truck ahead (or back) slowly for one tire rotation and have an observer note the number of driveshaft rotations. Compare the number of turns to the possible ratios. You can also do this by jacking up both rear wheels and turning them by hand.