I have a 1994 Chevrolet 1/2 ton 1500 PickupQuestion:  I am see oil slung on the under body both sides and the top near the rear differential, what could the possible problem be?Answer:  From you description you probably have a Pinion Seal Leaking.  Generally you will see fluid on the driveway, (moisture around where the drive shaft enters into the differential) suggest you only drive your vehicle to the shop and have an experience differential mechanic change the pinion seal and re-set the torque on a new crush sleeve.Incorrect setting of the torque on the pinion nut will lead to significant damage. What occurs is the pattern on the ring and pinion gear won't be correct and you will damage the ring and pinion set (within a few miles), then you must replace the ring and pinion (they're manufactured and sold in sets, they must be replaced in sets) Leaking pinion seals can allow the pinion nut to back away also it has been report on several occasion that the driveline can fall out! Find a video that gives more information on pinion seal changing at http://www.provenforce.com/rear_end/rear_end_questions_answers.html.  you can see the other rear end differential parts we can supply on our site.