Oil on the side of the differential case can be a sign of leaking seals. A small amount isn't anything to worry about, but you should check the fluid level to be sure it's not running dry. Just remove the fill plug and put your finger in the diff. If you can touch oil or feel it, then you should be ok, but adding a little extra never hurts, as too much oil will simply flow out the fill plug.
Before changing the seals try a seal softener. The seal softener is any commercial engine stop-leak product from your local auto parts store. Use as directed for engine leaks. CAUTION: Do not use auto transmission stop-leak, as the fluid is less compattable with engine and gear oils! For the tansmission/front differential use half the container. For the rear differential use one third of the container. Most leaks will dry up after about 500 or 1K miles. Clean the underside so you can accurately monitor the leakage. This $2.00 and ten minute fix is surely worth a try before tackling seal replacement.
Replacing Half Shaft Seals (not drive shaft seal)
For both seals, you need to unbolt the CV joint for the half shaft only at the diff, then push the half shaft up and out of the way so that you can pry out the seals. No need to remove the tires or suspension components. To remove the bolts, you'll need a special tool which looks like a hex key but it is actually an 8mm 12 point key as opposed to the 6 point hex keys. Then you have to remove the bolt that holds the half shaft flanges to the diff. These bolts are about 3" long and are removed with a 6mm allen key or hex key. This is easy, requiring only two of the CV joint bolts and a pry bar between the two as a wedge to prevent the flange from turning. These bolts aren't torqued in too tight anyway.
One thing to be VERY careful of: there is a shim which looks a lot like a very flat, thin washer placed between the driver's side flange and the diff gears. DO NOT LOSE THIS SHIM! They vary in thickness from car to car, so it's not like you can just buy the replacement part. The passenger side can safely be removed without worries, as there is no shim on that side.
The seal on the driver's side can be difficult to remove because it is large (approximately 3" OD and 2" ID). The seal on the passenger side is a cake walk (approx. 1.75" OD). To install the seals, use a short piece of pipe with a slightly smaller od than the seal.
Once the new seals are in, put the flanges back in and tighten them up to about 20 lb-ft of torque using, once again, two of the CV joint bolts and a prybar to prevent the flanges from turning while tightening. DON'T FORGET THE SHIM ON THE DRIVERS SIDE!!! Now you can put the half shafts back in place and tighten up their 6 bolts to 33 lb-ft of torque. Also, it's a good idea to try (tough one though) to add grease to the cv joints while they are exposed and add loctite blue to the bolts that hold the cv joints to the flanges.
If you do decide to replace the diff seals, do both, not one, as it's almost guaranteed the one that wasn't changed will leak in a very short time, besides, they are pretty affordable from the dealer. Also use GL-5 hypoid oil (75w90 synthetic diff oil) to replace the fluid.
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