DML FAQ : Transmission and Gears : How can I tell if I have an open or limited slip differential?
Jack up the rear of the truck so that both tires are off the ground. Put the transmission in neutral and spin one of the tires by hand.
If the other tire spins in the same direction, you have a Sure Grip (limited slip) rear. (Other possibilities include an aftermarket limited slip, a locker or a spool.)
If the other tire spins in the opposite direction, you have an open rear end.