Was under the car today, a 'class 1' (wet with oil and drops forming) leak on the diff at the seal where driveshaft hooks in; oil all around so its spinning off while driving too. What kind of oil should I put in it, where is the fill plug and how do you check the oil level? My truck was easy enough, take out the plug, stick your finger in the hole, if the oil is level you are good, if not add gear oil. The only plug that looked obvious was on the bottom side of the diff. If I take it out all the oil would follow I think!. Also, can a 'crunchy' sound which I thought was my wheel be the diff or the driveshaft (ouch, big ticket items now).