Fortunately with an "OPEN" rear end, things are easier and less expensive. One possibility is defects in the spider gears (rust or chunks missing for example) Some basics to study (look at image):
1)Axle Shaft.
2)Ring Gear.
3)Side (axle) Gears.
4)Spider Gears.
5)Housing.
6)Pinion Gear.
7)Pinion Shaft.
The pinion shaft is rotated by the Drive shaft (engine power).The Pinion gear rotates the ring gear .When the ring gear rotates ,both spider gears attached to the ring gear rotates.Spider gear rotates both wheels individually and freely. where the final power is transferred to shaft gears, which finally turn wheels.
Spider gears allow the "outside" wheel when turning in a radius, to travel further than the "inside" wheel. A "Locking" rear end uses other components (in place of the spider and side gears) to allow the difference in the distance a tire travels while in a turn.