The differential is really simple but yet a feat of engineering. A pinion gear supplies power to a ring gear that goes through a set of spider gears and turn the wheels. The spider gears are the key to its performance as they allow the outside wheel to turn faster and the inside to turn slower on corners. A Differential has to be properly set up as there is nothing more annoying than a ring gear whine on the highway or a clunk when put in gear. There are several tricks that can help these problems without a complete overhaul. If the differential is noisy IE a high pitched whine there are 2 solutions one is to add GM posi traction additive to the gear lube and the other is to add a high pressure lubricant JB makes an excellent one. These will help lubricant adhere to the ring gear so it is not dry when it comes down the back side. A clunk is another matter if you have a crew cab pickup or a large maxi van you may never completely cure the clunk as the drive shaft is too long. One cure is to put a small dent in the shaft this removes some of the harmonics that the longer shaft creates. Be careful you can cause a vibration by doing this. The other is to drill holes front and rear and fill with insulating foam this method seems to have the best result. I worked in a dealership when the Dodge maxi van first came out and they had terrible drive train problems. Drive shaft angle was a critical adjustment in many of these. Switched complete rear end assemblies to cure a vibration once and in the end neither van had a vibration. When new differentials use the material in the lube to self polish the gears so if you are going to switch to synthetic run the original for several thousand miles before switching. The newer the vehicle the less important this is a machining has improved over the yearsDo you have posi and what is your ratio? Jack up both rear wheels and turn one if the other side turns the same way you have posi if they turn in opposite direction then you don’t. To find gear ratio check what different options your vehicle has then mark the drive shaft and a rear wheel with a chalk line. Turn the drive shaft one full turn and the number of turns that your wheel makes is the ratio. This is not accurate but if you know what ratios your vehicle came with you can come close enough to find the correct ratio.