Just look on the badge on the inside door where it meets the frame of the truck when closed. There will be a code under the bracket called "axle". Tell us the code and that will tell you what gear ratio your axle is and whether or not it is limited slip or non-limited slip.
you could also jack up the back of the truck, put it in neutral, and turn one tire. If the other tire spins in the same direction, you have it, and if it spins in the opposite direction, you don't. But the door code is definitely easier.